Polymorphism is a rare magical ability that few mages possess. Philippa Eilhart is one of the few capable of it. She can transform herself into a large grey owl at will, and does so in Oxenfurt when she, Geralt, Shani, and Dandelion have a run in with some of the less friendly residents.
Mages are not the only group who can polymorph, several species are capable of doing so naturally. Dopplers do so naturally, and can assume any form they want.
Golden dragons can also polymorph innately into any form they desire: most notably the dragon Villentretenmerth, who notably took the form of Borch Three Jackdaws as a human. Some dragons also can polymorph, although at a limited capability, if said dragon is cross-bred with a golden dragon: Villentretenmerth's offspring, Saesenthessis can also polymorph, but to one human form, known as Saskia.
Higher grades of vampire can also polymorph: some Bruxa and Alp can assume a humanoid form to conceal their natural form; although more intelligent Bruxae, alongside Higher Vampires can maintain polymorphy in a longer state and can easily blend in with society.