Cursed Sapling

Normally it walks along on its roots, which is charming and slow. Get far enough ahead of it and it launches itself at you trailing fire, straight through walls, and if there is nowhere solid to land when it arrives it just circles you in the air instead. That flying state is the one you see most while caving, since there is rarely a floor where you want it.

While it is airborne it throws off a serious amount of light, better than most pets manage, and it will often reveal a cave mouth before your torch does. The cost shows up at the water. It splashes constantly and the sound it makes on impact is the same one a bite makes, so fishing by ear becomes impossible with it out. Either watch the bobber, keep a Sonar Potion running, or dismiss the pet before you sit down at the lake.

Mourning Wood carries it during the Pumpkin Moon, and the odds climb steeply with the wave number. That makes it a reason to push deep into a moon rather than farming early waves for tokens, since the late waves pay for the pet and the tree drops at the same time.