I've never really worried about it, I've created many new critters, some have been named the same in other supplements as I would find out later, but I didn't care, I used them how I made them in the first place
In my longest running game, I had drow that were very different from the books, mostly because I'd run across the concept way before the books detailed them in D&D terms. (yes, the game started before the old Giants series and such) I liked using my drow as foes since they could have classes and were clever with items, which always disintegrated in sunlight
This meant I could go nuts with magic items and powers, knowing the PCs could not use them for long.
I have been inspired by many outside sources, but usually created my own version of things. I put in a bow inspired by the bow used by Hank the ranger from the D&D cartoon, but it didn't have all the abilities his did. I had intelligent weapons with drawbacks, and since I made magic items very hard to make (most had been created before and during the Age of Chaos) PCs couldn't really make them without removing themselves from the game for a really long time.
Role-Aids had a lot of great books that 'reinvented' standard monsters, trolls had sub-races with different cultures and such, goblins, orcs and kobolds, standard D&D monsters, had skills and abilities beyond the Monster Manual.
People knew that in my game, you couldn't count on anything the Monster books said, I made them consistent within my game, but not with the books
Mira (It was recently discovered that research causes cancer in rats)
