None of those pokemon have a pink thing on their chest, or white also having presence on the color scheme (...What?) that, along with the apparent shape the head seemed to have and the horrible video quality, gardevoir wasn't completely improbable at first sight; more probable than scyther and reuniclus, for sure (scyther is nothing like it, and it's actually a less blue and more yellow green than gardevoir, and reuniclus is already a 3-stage line, and that thing is clearly not a mega evolution, not to mention their differences in shape, and that reuniclus' color is more true green than gardevoir's).
It being connected, somehow, to Zygarde is the most likely thing, once you take into account the color and the serpent-like body with a triangular head.
Gardevoir's green (hair) is #A0D09E
Scyther's green (skin) is #9BCF8C
Reuniclus' green (cytoplasm) is #8EC79C
Zygarde's green is #90B63D
Boogermon's green, from the shaky camera picture, is #AAFAA6
Gardevoir's heart's color is #E6787B
Reuniclus' hand organelles' color is #E19494
I can't get a clear read on Boogermon's core, it comes up as #C0926F, which is a shade of brown. However, we can all agree that it's red/pink-ish. The magic of color theory is that you register hues and shades based on the colors around whatever you're looking at.
Reuniclus's RGB values for his cytoplasm are actually all slightly lower than Gardevoir's values, which means it's just a scosche less saturated than Gardevoir's hair (so Reuniclus' cytoplasm is the less green of the two). Scyther's skin has less Blue than Garde and Reuni, which is why it looks more yellow-ish. Boogermon is almost completely green with halfway Red and Blue values, and it looks severely more neon than the others. Reuni's red hand organelles have more rounded-out values, whereas Garde's pink-ish hue has less Blue and Green. If you remove their reds from the rest of them, they might look duller then you normally think because a color slightly greyed out next to its complimentary color makes it look even brighter than it really is.
I think you should read over your comment about contrasting Gardevoir from Reuniclus because only one of them has a three-stage evo family, btw...
If I were going to explain my view, it resembles Reuniclus more than Gardevoir, not counting the use of green and red, for the reasons that it has a fat, rounded head in relation to its tiny body and you can see a reddish inner component of itself within its body that doesn't protrude out of it. If anyone's going to think up a rumor about a prevo to an already complete 3-stage evo line, in this case, I believe Reuni would be more likely than Garde.
