All Regenerating creatures are supposed to regain HP at the end of each round (though how much isn't entirely clear, since this ability is bugged and doesn't work). Interesting trivia about Regenerating creatures: the Flame Sword is supposed to be effective against them. (D&D's influence rears its ugly head again!)
All Regenerating creatures gain back 5% of their max HP at the end of each round. With the exception of Warmech, this is usually a trivial amount that can be safely ignored. It is surprising (unless you're a D&D player) that the Flame Sword deals extra damage to Regenerating creatures, but this fact really doesn't come up all that often.