What if I hatch a bunch of Yamask eggs? Is that mass genocide?

On a serious note (compared to my last post and response to your question, that is), it actually depends on whether or not you believe wholeheartedly in the folklore associated with yamask. Given the frequency of a yamask birth, the consensus among pokémon researchers is that it’s statistically unlikely that yamask are the spirits of deceased humans. Rather, it’s more likely that yamask are simply pokémon attached to mask-like objects that, through our tendency to see ourselves in literally everything (https://en.wikipedia.org/wiki/Pareidolia), human beings have interpreted to resemble human faces.

The same can be said for phantump, actually. They’re not literally the lost souls of human children; we just created folklore that says they are because their appearances and cries resemble those of distressed toddlers. The only difference is that Kaloseans realized this rather quickly because trevenant decidedly do not resemble distressed toddlers on any level.

How common is it for people to try to decorate Trevenant up as a Christmas tree and how well does that tend to go? Just saw a drunk guy getting chased by a Trevenant with ornaments on it.

It depends on the region, anonymous. In Kalos, it’s rather common, especially if the trevenant is tame. Tame trevenant tend to be raised from phantump if not exposed to Kalosean culture at an early age, so the act of decorating them either invokes memories of their past lives or allows them to participate in a tradition they’ve seen their humans prepare for months in advance. Christmas in general is rather big in that region, so even the wild pokémon of Kalos are familiar with its imagery (which in turn makes them a little more accepting of it).

On the other hand, in regions where it’s less common to decorate pokémon or celebrate Christmas to begin with (such as in the cases of Alola and Hoenn, respectively), it’s not that common, no, but it’s a practice brought in by foreigners. Such as your neighbor, perhaps.

Phantump and Trevenant

Phantump
The Stump Pokémon
Type: Ghost/Grass
Official Registration #: 708
Entry: This small spirit pokémon is actually a stump possessed by the soul of a child who died while lost in a forest. In light of this, the author once attempted to convince the Pokémon Association to ban the use of this pokémon in battles … until it was found out that in many cases, this is a dream come true for the child-turned-phantump, and to make a long story short, the author spent the next few weeks questioning his life decisions and the world as a whole.

Trevenant
The Elder Tree Pokémon
Type: Ghost/Grass
Official Registration #: 709
Entry: The evolved form of phantump, via trading. Using its roots as a nervous system, trevenant has the power to tap into and manipulate the trees around it. It most often uses this ability to trap those who harm the forest, but every so often, it traps and confuses children as well, who subsequently die and become phantump. Because no one is more brutal towards children than the spirits of other children, apparently.