Maybe something like a "Recommended Game" feature, next to some of the monsters. Several, like the Satyr, would work well in Doom, Heretic and Hexen, but the Rail Arachnotron is pretty Doom-specific, and the Knight Archer fits Heretic and Hexen better.
Of course, people (like CodenniumRed did) could just go through the Beastiary and decide for themselves whether or not a monster fits in whatever game.
@Tormentor: You haven't seen all of the monsters, yet. I'm afraid some, like the Fire and Ice Imps from Hexen 2 monsters, are pretty bland, even though they emulate the original monsters perfectly.
@Cutty: Already worked it out in Decorate-only (I hate using ACS in monster resources

), though thanks for the offer.