So you've caught the first little apple of your eye on some random planet. The little bugger, it does all your bidding, and kills for you unconditionally, yet what does it gain? As time roles by, you begin to notice your little freakish alien thing grow up to be the terrifying world ender you always dreamed it would become! Pretty soon, you have your own miniboss of your very own! Congratulations, this monster is every space traveling genocidal maniac's dream come true! You're almost afraid to go out in public! As per my suggestion, I suggest mobs change form after ranking high kill counts. It'd make pokemon so much more fun to play on starbound!
Meh, it would be cool but there should be a way to stop it, some people want to keep their cute little monster but still have it be useful and powerful.
dunno, would be hard too make a convincing "visual" progression for randomly generated monsters (unless the parts themselves are what progress), might be better off with something like a machine that can be used to change a monsters parts, abilities and stats manually.