Never paid much attention to fakemons, but as for unused Pokemon, I'll vouch for Honouguma, as long as it's completely made of fire and an overpowered piece of crud that years of imagination have led him to be in my head (it's not like he exists anyway, so...). I'll also take that unused sea lion thing, Kurusu. And if Goomy and Chikorita both didn't exist, Happa, because it looks like the end result of breeding both together.
Then again, maybe I don't want Honouguma. They'd probably make him weak and cuddly, and that'd spoil my fun. And this is coming from a person who prefers cuddly to dangerous Pokemon.
iRL, I have reservations for the same reason most people have stated. I can't see the sudden introduction going over well in any scenario. Even a Rattata could be the downfall of humanity (ok, probably not but...). Assuming they always existed... all of them? I mean I guess if they were always here, that's sort of a non-starter of a conversation.
Assuming I can be a bit selfish for a minute, though, going back to the first scenario, I'd probably have Nidoran Female, Nidorina, Aromatisse, Goodra, Miltank and Vulpix around on the condition that they act decently, and I'm certain they can. Heck, I'll probably go Meowth on them and we'll spend countless months training them in the pronounciation of the English language and its nuiances. Can't be any harder than training a human adult. If Pokemon want to be treated as equal partners, then they have to earn their keep and at least learn how to cook and clean up after themselves (by which I mean picking up toys, shedded fur/quills/feathers (I'm gonna make Aromatisse get good with a vaccuum, all that molting >:() and such, not that other way. Though toliet training is a thing...). Not sure how much I want a soup that Goodra had a hand in making. How much of that soup is her in there somewhere? x_x And if they go out in the world and people give em flak, well... they have me and each other. ;m; Cruel, cruel world...
I'd also have to have about a zillion antidotes on me for Nido hugs. Sheesh, I hope I get used to the idea of stabbing myself in the arm with a syringe if I hold them the wrong way.