9the pet
The Baby Truffle puts out a faint blue glow, which makes it one of the rare cosmetic pets that does a small job on the side. It is nowhere near a real light pet, but in a dark room it is enough to make out your own feet.
how it follows you
It does not chase. Get far enough ahead and it simply blinks out, then reappears where you are, usually once you touch the ground. It looks strange the first few times, but it means you never lose it and it never gets stuck on terrain the way flying pets sometimes do.
what it costs you
The Truffle sells it, so a surface mushroom biome and a house he will live in are the real price. That is a lot of setup for a pet, and by the time you have him there are better things on his shelf to spend on first.