Pretty much anything that came by Earth. Our own space programs (the official ones like NASA etc) don't have anything that can even get out of the solar system yet, but we do have enough tech out there that other species are noticing us and coming to see what's going on here. London pretty much collected anything they could find that got left behind, figured out how it worked and riffed off it. In Cardiff there was a rift in space-time though so we'd get single individuals that had slipped through - that's where the pterodactyl came from, too. There was also a stable population of what we call Weevils in the sewers that we regularly had to deal with if one wandered out. They're humanoid, semi-sentient, but very aggressive when they're out of their element and full of sharp teeth.