A hamrammr (from old Icelandic literature) is a werecreature that shifts into the form of the animal it has most recently eaten. Its strength increases with each animal that it consumes.
Ireland and Scotland
The Selkies are seals that take off their skins to become human. Dark-haired Celts may have their origins explained via the Selkies. They are usually helpful creatures who watch over fishermen. Kelpies are a kind of shapeshifting sea horse.
Púca and some other Celtic spirits and Síde (fairies) can change their form at will and typically pose as animals or loved ones. Leprechauns turn into hideous creatures to scare you into releasing them when captured.
Norway and Sweden
The eigi einhamir or "not of one skin" has the ability to change into a wolf by wearing a wolfskin. There is also the ulfheobar (wolfskin), which is usually lumped in with berserker.
Leszi are spirits of Slavic mythology, capable of changing into any creature or plant.
The lubins or lupins look like wolves, but can speak human languages. They are very shy.