Mammals
A nail is homologous to a claw but is flatter and has a curved edge instead of a point. A nail that is big enough to bear weight is called a 'hoof' (see also Horse hoof. However, one side of the cloven-hoof of artiodactyl ungulates may also be called a claw).
Every so often, the growth of claws stops and restarts, as does hair. In hair, this results in the hair falling out and being replaced by a new one. In claws, this results in an abscission layer, and the old segment breaks off. This process takes several months for human thumbnails. Cats are often seen working old unguis layers off on wood or on boards made for the purpose. Ungulates' hooves wear or self-trim by ground contact. Domesticated equids (horses, donkeys and mules) usually need regular trimming by a farrier, as a consequence of reduced activity on hard ground.
Many predatory mammals have protractile claws that can partially hide inside the animal's paw, especially the Felidae, where almost all of its members have fully protractible claws.