Polydactyly is seen in other species. There are several forms of polydactyly in laboratory mice; these are used to study how limbs develop. Some cause simple duplication or bifurcation of thumbs while others cause multiple abnormalities by disrupting how the embryo develops. Some cause mild effects when heterozygous, but have severe (or even lethal) effects when homozygous. Many laboratory polydactyl mice result from mutations that disrupt limb formation in general, causing polydactyly, syndactyly and underdeveloped, missing or twisted long bones in the legs. A severe form called "Doublefoot" (Dbf) is lethal when homozygous and disrupts limb formation. One effect of this is triphalangeal preaxial polydactyly (like Twisty Cats), 6 to 9 toes on fan-like feet, dwarfing, twisted and/or thickened limbs, short or missing tibia/radius, kinked tail, flattened ribcage and short, bulbous head. Dbf can cause forked digits in the thumb/big toe position and floating digits (not connected to the wrist) between normal digits. Researchers have discovered that different mutations can cause similar physical effects by giving the embryo garbled instructions on how to build a limb; though each mutation is a different type of garbled instruction the end result is often very similar. Other gene mutations have no other effect except to give a mouse one or more extra toes; these are less well reported and usually less interesting to researchers as they don't demonstrate the wider ranging effects of certain genes.

A number of ancient shepherding dog breeds exhibit polydactyly and it is considered an unremarkable trait. Some have single or double dewclaws on the front and/or hind paws. Breeds where the polydactyly trait is encouraged are those bred for working on snowy or uneven ground where extra grip is required. Dewclaws are frequently removed in non-working, non-show pet dogs as they may snag, hence canine polydactyly is often overlooked. Some canine breed standards (depending on which official body the dog is registered with) require the dewclaw(s) to be present in show specimens while others permit their removal.

The Norwegian Lundehund must have at least six toes and eight pads on the fore paws; five toes must rest on the ground. On the hind legs there must be at least six toes, four of which must rest on the ground. The toes have extra joints to aid it in its traditional job of puffin-hunting, for which it needs to climb cliffs. In the Beauceron, the double hind dewclaws should form thumbs and be close to the foot to provide a larger weight-bearing surface. The Catalonian Sheepdog's double hind dewclaws must be joined together and joined to the first (inner) toe by a membrane (webbing). In the Iceland Shepherd Dog, hind dewclaws are essential and double dewclaws are preferred. The Briard's double hind dewclaws must be located close to the ground. In several other breeds, hind dewclaws are permitted e.g. Great Pyrenees (double), Portuguese Sheepdog (single/double), Cao Fila de Sao Miguel (single), Cao de Castro Laboreiro (single/double on both front and hind feet), Saint Bernard, Pyreenees Mastiff (double preferred over single), Estrela Mountain Dog (single/double), East Siberian Laika, Anatolian Shepherd Dog (double).

In horses, polydactyly has occasionally appeared in the form of small supernumerary digits terminating in hooves either side of the main hoof; these resemble the horse's evolutionary ancestors hence older texts refer to such mutations as atavism (throwback hrowback to ancestral stock). According to Suetonius, "[Caesar] used to ride a remarkable horse, which had feet that were almost human, the hoofs being cleft like toes. It was born in his own stables, and as the soothsayers declared that it showed its owner would be lord of the world, he reared it with great care, and was the first to mount it; it would allow no other rider." Caesar's polydactyl horse is shown left (artistic licence has made it into a unicorn). Bucephalus (the mount of Alexander the Great) was also described as a polydactyl. More recently, I have seen photos of a Thoroughbred showing this trait. The extra digits (one either side of the normal hoof) were non-functional, about 3 or 4 inches (7-10 cm) long and terminated several inches above the ground. They ended in small hooves, resembling the Merychippus.

O C Marsh (1892) described "Recent polydactyle horses" in the American Journal of Science. A normal horse has only digit III, this being developed into the single hoof. Marsh described a "horned horse from Texas" which had three toes on the hind legs (digits II, III & IV) and two toes on the fore legs (digits II & III). This is similar to the feet of the ancestral Merychippus. According to Marsh and a later German study, two-thirds of polydactylic horses were the result of a duplicated digit III while the rest were due to fully developed versions of the normally vestigial splints of digits II and/or IV.

In humans, supernumerary fingers and toes are generally removed early in life for cosmetic and/or safety reasons, for social acceptability and because clothing and utensils are designed for five-digited people. In developing world and third world countries the digits are more likely to remain in situ. Depending on prevailing beliefs, such individuals may be seen as lucky, unlucky, blessed or cursed. There are a number of cases of human polydactyly and syndactyly recorded in early medical texts. The following is adapted from "Anomalies and Curiosities of Medicine" written in 1896 by George M Gould & Walter L Pyle.

Anomalies of the Feet. Bull gives a description of a female infant with the-left foot double or cloven. There was only one heel, but the anterior portion consisted of an anterior and a posterior part. The anterior foot presented a great toe and four smaller ones, but deformed like an example of talipes equino-varus [a form of club foot]. Continuous with the outer edge of the anterior part and curving beneath it was a posterior pan, looking not unlike a second foot, containing six well-formed toes situated directly beneath the other five. The eleven toes were all perfect and none of them were webbed.

Anomalies of the Hand. Murray cites the instance of a woman of thirty-eight, well developed, healthy, and the mother of normal children, who had a double hand. The left arm was abnormal, the flexion of the elbow imperfect, and the forearm terminated in a double hand with only rudimentary thumbs. In working as a charwoman she leaned on the back of the flexed carpus. The double hand could grasp firmly, though the maximum power was not so great as that of the right hand. Sensation was equally acute in all three of the hands. The middle and ring fingers of the supernumerary hand were webbed as far as the proximal joints, and the movements of this hand were stiff and imperfect. No single finger of the two hands could be extended while the other seven were flexed. Giraldes saw an infant in 1864 with somewhat the same deformity, but in which the disposition of the muscles and tendons permitted the ordinary movements.

Absence of Digits. Maygrier describes a woman of twenty-four who instead of having a hand on each arm had only one finger, and each foot had but two toes. She was delivered of two female children in 1827 and one in 1829, each having exactly the same deformities. Her mother was perfectly formed, but the father had but one toe on his foot and one finger on his left hand. Kohler gives photographs of quite a remarkable case of suppression and deformity of the digits of both the fingers and toes. A man who was recently exhibited in Philadelphia had but two fingers on each hand and two toes on each foot, and resembles Kohler’s case in the anomalous digital conformation. [Also shown is] an exhibitionist with congenital suppression of four digits on each hand. Tubby has seen a boy of three in whom the first, second and third toes of each foot were suppressed, the great toe and the little toe being so overgrown that they could be opposed. In this family for four generations 15 individuals out of 22 presented this defect of the lower extremity. The patient’s brothers and a sister had exactly the same deformity, which has been called “lobster-claw foot”. Hutchinson exhibited a photograph showing the absence of the radius and thumb, with shortening of the forearm. Conditions more or less approaching this had occurred in several members of the same family.

According to Annandale, supernumerary digits may be classified as follows:-
(1) A deficient organ, loosely attached by a narrow pedicle to the hand or foot (or to another digit).
(2) A more or less developed organ, free at its extremity, and articulating with the head or sides of a metacarpal, metatarsal, or phalangeal bone.
(3) A fully developed separate digit.
(4) A digit intimately united along its whole length with another digit, and having either an additional metacarpal or metatarsal bone of its own, or articulating with the bead of one which is common to it and another digit.

Superstitions relative to supernumerary fingers have long been prevalent. In the days of the ancient Chaldeans it was for those of royal birth especially that divinations relative to extra digits were cast. Among the ancients we also occasionally see illustrations emblematic of wisdom in an individual with many fingers, or rather double hands, on each arm.

Hutchinson, in his comments on a short-limbed, polydactylous dwarf [a premature stillbirth] which was dissected by Ruysch, the celebrated Amsterdam anatomist in 1668, writes [...] "The point of extreme interest in the present case is that this dwarfing of the limbs is associated with polydactylism. Both the hands have seven digits. The right foot has eight and the left nine. The conditions are not exactly symmetrical, since in some instances a metacarpal or metatarsal bone is wanting; or, to put it otherwise, two are welded together [...] This occurrence of short limbs with polydactylism seems to prove conclusively that the condition may be due to a modification of development of a totally different nature from rickets."

Forster gives a sketch of a hand with nine fingers and a foot with nine toes. Voight records an instance of 13 fingers on each hand and 12 toes on each foot. Saviard saw an infant at the Hôtel-Dieu in Paris in 1687 which had 40 digits, ten on each member. Annandale relates the history of a woman who had six fingers and two thumbs on each hand, and another who had eight toes on one foot. Meckel tells of a case in which a man had 12 fingers and 12 toes, all well formed, and whose children and grandchildren inherited the deformity. Mason has seen nine toes on the left foot. There is recorded the account of a child who had 12 toes and six fingers on each hand, one fractured. Braid describes talipes varus [club foot] in a child of a few months who had ten toes. There is also on record a collection of cases of from seven to ten fingers on each hand and from seven to ten toes on each foot. Scherer gives an illustration of a female infant, otherwise normally formed, with seven fingers on each hand, all united and bearing claw-like nails. On each foot there was a double halux and five other digits, some of which were webbed.

The influence of heredity on this anomaly is well demonstrated. Réaumur was one of the first to prove this, as shown by the Kelleia family of Malta, and there have been many corroboratory instances reported; it is shown to last for three, four, and even five generations; intermarriage with normal persons finally eradicates it.

It is particularly in places where consanguineous marriages are prevalent that supernumerary digits persist in a family. The family of Foldi in the tribe of Hyabites living in Arabia are very numerous and confine their marriages to their tribe. They all have 24 digits, and infants born with the normal number are sacrificed as being the offspring of adultery. The inhabitants of the village of Eycaux in France, at the end of the last century, had nearly all supernumerary digits either on the bands or feet. Being isolated in an inaccessible and mountainous region, they had for many years intermarried and thus perpetuated the anomaly. Communication being opened, they emigrated or married strangers and the sexdigitism vanished. Maupertuis recalls the history of a family living in Berlin whose members had 24 digits for many generations. One of them being presented with a normal infant refused to acknowledge it. There is an instance in the Western United States in which supernumerary digits have lasted through five generations. Cameron speaks of two children in the same family who were polydactylic, though not having the same number of supernumerary fingers.

Smith and Norwell report the case of a boy of fifteen both of whose hands showed webbing of the middle and ring fingers and accessory nodules of bone between the metacarpals, and six toes on each foot. The boy’s father showed similar malformations, and in five generations 21 out of 28 individuals were thus malformed, ten females and 11 males. The deformity was especially transmitted in the female line. Instances of supernumerary thumbs arc cited by Panaroli, Ephemerides, Munconys, as well as in numerous journals since. This anomaly is not confined to man alone; apes, dogs, and other lower animals possess it. Bucephalus, the celebrated horse of Alexander, and the horse of Cesar were said to have been cloven-hoofed.