Quote Originally Posted by LocoColt04
If you come across any of those creatures, yes. Off the top of my head I can't give you a list of any creatures that appear in both worlds, but I know there are a handful. All others, however, are gone once the WoB is gone, thus rendering it impossible to get their Rages.

A lot of Rages do the same things though, so it's not too big of a deal if you miss, unless you're doing it for completion value.
I just want to state for the record that this is incorrect.

All the monsters you fought in the World of Balance are still found in the World of Ruin after the change over.

I had a guy from gamefaqs who made an FAQ there called The Rage Guide answer a few questions I had, and then reasked him this, and he said I could still get them then after the world changed over.