Mmmm.... the whole copyright thing is a tricksy business and no mistake. As regards the owlbear I'm not sure that saying " hey, I nicked it off of Everquest" puts you in the clear as :
1] Everquest might then go "Oi..thats ours that is "
2] Everquest might have it under license from WOTC in the first place
Bugbear is pretty safe as it's an old English legendry term
Thing is though, you have to ask have WOTC respected everybody's copyright ? Consider the humble Orc. Now in modern day RPGing Orc is shorthand for ' nasty, brutal, tribal thingy used as sword fodder for steely eyed heroes '. And that is pretty much straight out of Tolkien. Let's face it Tolkien more or less gave the term Orc to the modern world ( it was an archaic, forgotten English term before that ) So did TSR back in the 70's and later WOTC get permission from the Tolkien estate to use the term in such a fashion ? If not where does that leave them if they pick up someone for using some of their stuff ?
Murky waters Watson, murky waters indeed.