Hm, I suppose capture isn't the right word. At least, not in the traditional sense. But... it certainly seems like it's possible to control a legendary, at least for a short amount of time. I've seen it happen before.

[... It didn't really end all that well, and cause WAY more chaos than a bunch of weird mashed-up pokemon running around is doing, but...]

That I have no idea how to answer. I've read up on a few of the legendary creatures of this region in particular, but it doesn't seem like the ones that once lived here could be doing something like this. But... that doesn't leave out the possibility that something from a different region has shown up here.