Companion Cube
'Susceptible to lava!'
Will never threaten to stab you and, in fact, cannot speak
1the pet that hurts you
It hops along behind you like any other pet until the light goes out entirely. In complete darkness it starts stabbing, and it does not need to be beside you to reach. The hit is tiny but it counts as real damage, so it triggers on-hit effects and it can finish off a character who was already nearly gone. There is a death message written specifically for that, which is the joke.
the price
It is the most expensive thing the Traveling Merchant will ever put in front of you, and the most expensive item in the game full stop. Nothing about it is practical. You buy it because you want it on your screen.
the sound gags
Drop it in lava and it screams with the Lunatic Cultist's death sound, and if you leave it there long enough it will occasionally reach for the Moon Lord's instead. It also passes gas at random intervals. The lava reaction is the punchline to the Portal reference the whole item is built on, since that is how the original game disposes of the cube, and the tooltip borrows the announcement insisting the thing could never threaten to stab anyone.