According to old tales, these Pokémon are stumps possessed by the spirits of children who died while lost in the forest.
– Pokémon Y Pokédex
Phantump’s Y Pokedex entry is pretty creepy, though I do find Phantump kind of cute.
It (and its evolution) have the unique egg group combination of Grass and Amorphous (for some reason, its counterpart Pumpkaboo is only in the Amorphous group). When it was introduced, the Ghost/Grass typing was new. Now we have 6 Ghost/Grass Pokemon (or 5 if you want to be technical, since Decidueye is Grass/Ghost)
Stat-wise it’s pretty slow, but has pretty good Attack for a first-stage evolution Pokemon.