Are there any non-3pp monsters with regeneration that have a CR of 3 or lower?
Not sure why this got moved to Rules. Isn't really a rules question...
I found the Sewer Troll in Monster Codex.
Any others? Are trolls and troll-like critters the only ones that have regeneration without going into the outer planes?
If you're willing to go fast healing instead of regeneration, that opens up a number of weak outsiders. Quasits, for example.
Sadly, it is the "Can't die as long as regeneration is functioning" part that I need, more than the hp recovery, so fast healing won't work.
I didn't want to give too many details in case my players look into this thread, but I guess I'll get better responses if I take the risk. Basically I want to give a lot of PC levels to a monster and have it act as a "big bad" for a campaign. The lower the initial CR, the more class levels I can give before it gets too tough for the party to handle. Trollhound doesn't work so well for that, and even trolls don't really have the base intelligence to be leaders (except of other trolls), so I'd have to throw on a template (boosting the initial CR even more).
You can also took a Ja Noi Oni
Kuwa has the adventage that he can cast invisibility, which is helpful for recurring villains
Perfect! Oni sound like exactly what I need. Might change the flavor a bit but nothing I can't work with. Thanks!