|Persian Cat Armour by Jeff de Boer | via Grace Elliot|
|Owl Armour | Legend of the Guardians|
Normal Familiars in AD&D (and no, that doesn't include the Panzerbjorne, unless you're Polish) have 1-4 hit points, and an Armour Class of 7. Should an animal familiar be killed their magic user loses twice the familiars HP - a dramatic, potentially death dealing loss to a low-to-mid level magic-user. In a world where Familiars are an every-day part of magic using cultures, and magic users are often found in perilous situations, animal armour just makes sense.
So, for example, a familiar wearing plate mail gets AC2.