Not to be curt, but if it’s injured, take it to a pokémon center. 

First and foremost, a pokémon nurse’s priority is on healing an injured pokémon. They won’t judge you or report you to any sort of government body or what-have-you for owning a pokémon they don’t recognize. If anything, their inability to recognize such a pokémon may present a bit of a challenge for them, but ultimately, if they don’t know what your pokémon is, they’ll treat it as best as they can anyway, based on their knowledge of your pokémon’s injury, type, and general physiological profile (i.e., mammalian, avian, or so forth). Furthermore, pokémon centers are hospitals. It’s highly unethical for anyone from any sort of authority to break in and take your pokémon away for this reason, and you’re generally protected by poachers by virtue of the fact that you’re in a highly public area with highly trained and extremely powerful nurse pokémon (as well as teams of trainers who use pokémon centers as meeting places and who don’t take well to thieves).

Second, I can’t offer care tips for legendary or mythical pokémon or the Ultra Beasts. It’s not that I don’t want to (although I’ve been very open about the fact that my policy concerning owning legendaries is “put that back where you found it or so help us all”); it’s more because these pokémon aren’t that well understood. In the cases of legendaries and mythicals, they’re so rare that there simply aren’t enough cases of human interaction with such pokémon in recorded history to know what their needs are. In other words, very, very few people have interacted with these pokémon, much less caught them, and those that caught them are generally extremely competent trainers (not that those who ask for pokémon care tips on this blog aren’t competent, of course). 

As for the Ultra Beasts, we also don’t know enough about them to understand what their specific needs are, but in their case, they’re simply too new and destructive to our world. Thus, all instances of Ultra Beasts have already been secured by the work of the International Police and the Aether Foundation, the latter of whom has only just started releasing their research on actual, captive specimens (as opposed to theoretical research based on brief sightings).

Third and related to the above, it’d be odd to jump to the conclusion that you’re dealing with an Ultra Beast, just because it’s not an easily recognizable pokémon. It’s possible (and, quite honestly, more likely) that you’re looking at a poison-type pokémon from a different region on Earth, perhaps one that isn’t yet logged in the National Dex program. “Small and purple” describes many poison-types, after all. (This, incidentally, is why I also can’t offer tips for general care. “Small, purple poison-type” covers quite a lot of ground, even if we didn’t consider any known legendaries or UBs, and how one would care for a venonat differs vastly from how one would care for a koffing.)

In short, if you’re worried about this pokémon’s minor injuries and general condition, don’t be afraid to take it to the pokémon center. The nurse there may even be able to help you positively identify what you’re dealing with, which in turn will make figuring out how to take care of it immensely easier.

Best of luck, anonymous!

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{Note from the mun: Like the last ask, this one is also among the newer batch. It’s just that Bill’s been getting a lot of questions about how to care for a legendary or Ultra Beast lately, and it might be worth it to note that various asks and entries have already established that both classes of pokémon are so rare that it’s very, very unlikely that an ordinary person would encounter them. Bill likewise only knows about them via second-hand accounts, theories, and folklore for the most part. Just to clarify! Carry on!}