The critters of a science-fantasy post-apocalypse game tend to be very fantasy-creature-like, but with a twist that one just doesn't find in low-tech fantasy.
Here is a quick-and-dirty guide to conversion.
Post-Apoc <-> Fantasy
Shoots Electricity from its Eyes <-> Breathes Electicity
Breathes Radiation or Breathes Plasma <-> Breathes Fire or Breathes Cold (depending on climate)
Breathes Toxins <-> Breathes Toxins (no change)
Eye Lasers <-> Death Gaze
Paralyzing Beams <-> Petrifying Gaze
Sonic Beams <-> Death-causing Wail
Turns Metal to Rubber <-> Rusts Metal or Disenchants
Controls Robots <-> Charm power or necromantic spells
Death Touch <-> Death Touch or Level Drain
Immune to Radiation <-> Magic Resistant
Immune to Poison <-> Same
Radioactive Aura <-> Damage Aura
High Tech abilities <-> Well-armed and equipped
It's a bit tongue in cheek but it should work most of the time. A medusa would be a snake-headed woman with paralyzing rays in, say, Gamma World, while a hoop would have either a disenchanting or rusting touch in AD&D. A GURPS Eye of Death would shoot laser beams or super-science death rays in a Thundarr inspired setting. A dragon in fantasy is basically Godzilla in post-apoc. You can pretty much grab any monster book and convert quickly back and forth. You don't need a whole lot of rule work, either - a 5d burning attack from eye beams can be 5d electrical breath. Or keep the damage, etc. the same and just change the special effects.
Have fun with this!