There are no new enemy types on Playthrough 2.5, unlike playthrough 2.0 which introduces things like Armored Psychos or Bullyrots. I think (but am not sure) certain enemies change names (pyro pete the invincible -> pyro pete the ultra invincible) but I've noticed nothing different about enemies whatsoever in Playthrough 2.5 besides the fact that their levels are higher (and the resulting difficulty increase).
If enemies do change names, it's certainly not as noticable in BL1's playthrough 2.0. For instance Baddass enemies are still Badass or Super Badass, no more Bad Mutha Skags etc.