Hostile mobs grow stronger the more you kill them. A global per-species kill counter raises HP, damage, speed and XP for every new spawn, with an RPG-style level label above each mob.
Zombie at Lv.17 (red tier), creeper at Lv.6 (yellow), enderman at Lv.1 (gray). The more you kill a species, the stronger every new spawn gets — with a color-tiered level label above each mob.