B/X Known World
There's a difference between a troll and a beholder. The troll is generic and not under copyright, the beholder is unique and under copyright. The SRD lists what WotC considers Product Identity. On that list are "proper names (including those used in the names of spells or items)" followed by a long list of specific locations and characters and it finishes off with "beholder, gauth, carrion crawler, tanar’ri, baatezu, displacer beast, githyanki, githzerai, mind flayer, illithid, umber hulk, yuan‑ti." Of that entire list, I think only beholders and illithid / mind flayers appeared in C1. I know displacer beasts and yuan-ti appeared in C2...but I don't remember if either were actually name dropped in the episodes.I'd need some proof of that, it seems a bit questionable. They still use plenty of terms and spells that are not in the basic rules.
There's also a huge difference between the level of scrutiny most DMs have to deal with and what Matt does on Critical Role. You can use whatever you want and call it whatever you want. No matter how much you violate someone's intellectual property, you're not going to get a call from anyone's lawyer. On CR the cast has to drink from unlabeled cups and they have to cover any logos because they're broadcasting to however many millions of viewers on a weekly basis. They have to follow much different IP rules than the rest of us.On the other hand, I change the fluff, names (if I even give the name) and descriptions for monsters all the time so that it fits my vision. It has nothing to do with legalities.