You said I was on the right track with my red theory, but the wrong subject. So is the answer a snake or other animal that naturally detects infrared light?

How about the enemy Heg? It's a snake, located in a cave (which would echo); the cave is in Cosmo Canyon (where it's always dusk).

The problem with this answer is it doesn't fit with either interpretation of line 3; as I understand it, line 3 means one of the two following things:

1. The enemy is unavoidable at some point, OR
2. It has a physical-based attack that you can't evade.