I think Catlemur's idea would definitely work if you didn't know the strength level of the creature you were killing:
So, for instance (and taking Catlemur's ratios), if you have a field full of 20 Ulbernauts- 2 would be weaker than usual (stats down by 40%), 12 would be normal, 5 would be stronger than usual (stats up by 40%) and one would be incredibly strong (stats up by 100%). Obviously once one is killed and respawns, there's a 10%/60%/25%/5% chance of it being one of the different types.
However, all of them would simply say
'Ulbernaut' with no ability to read in the description how strong it is (except I've heard there's a Brown Way identification spell that allows one to actually see the hit points, strength etc. but not sure if it's a rumour)
That's my take on it anyway, and I think it would be a great way to mix things up in the game and make it more interesting- especially when you come across that super-strong one and have to run away very quickly
Xirius