This can be:

As a result of congenital abnormality in a normally pentadactyl animal. Polydactyly is very common among domestic cats. For more information, see polydactyly. Polydactyly in early tetrapod aquatic animals, such as in Acanthostega gunnari (Jarvik 1952), one of an increasing number of genera of stem-tetrapods known from the Upper Devonian, which are providing insights into the appearance of tetrapods and the origin of limbs with digits. It also occurs secondarily in some later tetrapods, such as ichthyosaurs. The use of a term normally reserved for congenital defects reflects that it was regarded as an anomaly at the time, as it was believed that all modern tetrapods have either five digits or ancestors that did. Oligodactyly
Oligodactyly (from Greek ὀλιγο- oligo- "few") is having too few digits when not caused by an amputation. It is sometimes incorrectly called hypodactyly or confused with aphalangia, the absence of the phalanx bone on one or (commonly) more digits. When all the digits on a hand or foot are absent, it is referred to as adactyly. Ectrodactyly
Ectrodactyly, also known as split-hand malformation, is the congenital absence of one or more central digits of the hands and feet. Consequently, it is a form of oligodactyly. News anchor Bree Walker is probably the best-known person with this condition, which affects about one in 91,000 people. It is conspicuously more common in the Vadoma in Zimbabwe.
In biology, dactyly is the arrangement of digits (fingers and toes) on the hands, feet, or sometimes wings of a tetrapod animal. It comes from the Greek word δακτυλος (dáktylos) = "finger". Sometimes the ending "-dactylia" is used. The derived adjectives end with "-dactyl" or "-dactylous". As a normal feature
Pentadactyly
Pentadactyly (from Greek πέντε pénte "five") is the condition of having five digits on each limb. It is traditionally believed that all living tetrapods are descended from an ancestor with a pentadactyl limb, although many species have now lost or transformed some or all of their digits by the process of evolution. However, this viewpoint was challenged by Stephen Jay Gould in his 1991 essay "Eight (Or Fewer) Little Piggies", where he pointed out polydactyly in early tetrapods and described the specializations of digit reduction. Despite the individual variations listed below, the relationship is to the original five-digit model. In reptiles, the limbs are pentadactylous. Dogs and cats have tetradactylous paws but the dewclaw makes them pentadactyls. Tetradactyly
Tetradactyly (from Greek τετρα tetra "four") is the condition of having four digits on a limb, as in many amphibians, birds, and theropod dinosaurs. Tridactyly
Tridactyly (from Greek τρία tría "three") is the condition of having three digits on a limb, as in the rhinoceros and ancestors of the horse such as Protohippus and Hipparion. These all belong to the Perissodactyla. Some birds also have three toes, including emus, bustards, and quail.