So wait, if one was to make all semimega/megabeast critters have entity mentions, you'd have a bunch of them start living in a cave, have kids, and still qualify for being megabeasts that randomly go and try to kill you?
Or would that make them suddenly appear in one huge siege/ambush wave, or behave like normal megabeasts except you could have generated dragons arriving along the pre-existing history ones?
EDIT: Hurm. Looks like the MEGABEAST tag also implies they're unable to breed. The tags for it are there, but this gen'ed world shows that the megabeasts did act like normal civ's, and fought even each other, but
they're all most of them are dead without kids now.
EDIT2: However, a few of them did survive the worldgen. Because they're cave-dwellers, they don't show up in the embark screen list of civs though, so I'll have to play a bit to see whether or not I can get them to appear, and if they do, how do they function. SCIENCE!
EDIT3: Well, this is what happened.
Linky!
There wasn't any "THE MINOTAUR THIS-AND-THAT HAS ARRIVED", and they currently operate like normal thieves. Except with the building destroyer tags they have, they're currently smashing every piece of furniture I have into bits. Although now that I think about it, I did just copy the kobold entity tags. I bet I forgot the ITEMTHIEF tag there too. Although I imagine it wouldn't have changed the situation much, if at all. They'd still appear like normal enemies.