Beastpox

Also referred to as a primal change, beastpox is a temporary affliction that strikes visitors to the Beastlands.  For normal animals, it manifests as the desire to roam free and the ability to do so.  Animal companions, familiars, mounts, and pets alike immediately strike out on their own when entering the plane.  There is nothing an owner can do to stop an animal from leaving, but the animals always seem to return just when the owner is about to depart from the plane.

For sentient creatures (i.e., player characters), beastpox manifests as an animal characteristic that most closely relates to their personality.  A strong person might sprout hair like a bear or develop stripes like a tiger; a hearty person might sprout oxen horns or grow an elephant's trunk; a wise person might grow a lion's mane.  Lawful creatures are the most resistant to beastpox, while chaotic beings shows signs of it within a few hours of visiting the plane.  In either case, the affliction lasts as long as the person visits the Beastlands and a like amount of time once the person leaves.