40how the pet behaves
A carved spirit head that hovers over your shoulder and spins. What sets it apart from most pets is the follow logic. It does not blink to your position when you outrun it, it just drifts toward you in a straight line, so a fast character on hooks leaves it trailing behind and then catching up. Some people like that, some find it distracting.
buying it
The Witch Doctor only lists it while he is standing in the Jungle, and only in Hardmode, so a tidy surface town means moving him out and back. Giving him a permanent Jungle house is worth doing anyway, since several of his better lines work the same way.
trivia
The floating spirit is a fairly clear nod to Aku Aku from Crash Bandicoot.