Considering that Macaques, Vultures, Ravens, and such all have the exact same ITEM_THIEF behavior, they should all be clustered together.
Very true; they may not all be equally relevant, though, if more players have had to deal with macaques than vultures, or if undead ravens are more dangerous in general, etc.
What about megabeast and semi-megabeast alignment?
Hmm, well, I don't think they'd do much for an animal chart seeing as they're only ever anything other than chaotic and evil if A) you (the player) are rooting for the goblins and they attack along with a siege or B) they come as diplomats -- and I'm not even sure about either, I may be mixing up semimegabeasts, titans and demons (I have limited experience so far with any, perhaps because of the way I naturally tend to play, although eventually I'd like to do more of the risky underworld-conquering stuff).
On the other hand, they might fill a nice spot on the character chart for Fortress Mode in the place of the bogeyman from Adventure Mode. While I initially thought of demons, forgotten beasts are more likely to attack your fortress (barring your opening the underworld and finding all the demons), from what I've heard? And then again, do they even use different ASCII characters or a different color or anything? I thought they're both represented by &, but I've never seen one directly that I can think of (maybe happened by a demon lawgiver in Adventurer Mode and didn't think to get a picture or memorize the color). I guess more experienced players will have to fill out some of those details.