Vagabond

No one knows a vagabond's true shape, since it is only ever met wearing the body of a native creature of animal intelligence or better, rarely anything as clever as a player character race. It may build a body from nothing (an inky blot appears on the ground, swells into three dimensions and sharpens into detail, a terrifying sight if the form is a wolfwere), move into a fresh corpse and heal it of all ailments, or share a living body like a back-seat driver that can urge but never force. A ridden horse still will not leap a cliff it thinks unsafe.

Once in a form it is locked there, leaving only through psionic powers such as its beloved switch personality. Talking to one soon reveals something wrong: it lacks the host's speech, customs and manners, though it uses the body's attacks, defenses and movement, often in peculiar ways.

It fights as well as its borrowed body does. Every vagabond is psionic and wholly immune to mental attack or control that is not strictly psionic. If the host dies, the vagabond departs forever.

It lives wherever its form would, either among that creature's kind or alone. Two are never found together; each senses another within a mile and moves away when it can.

Vagabonds seem to visit the Prime Material Plane in search of knowledge, and they are endlessly nosy about even trivial or personal things. Invited along on an adventure, one is 90% likely to join and lend its considerable power to the group. One hiding in a character's war dog or warhorse (usually a true neutral host) makes a fine surprise. Good and evil vagabonds side with like-minded forces, helping them while studying their ways.