: that explains why I'd always run into shocking spheres in labyrinth, ghouls
: in low flying nightmares, and invisible wraiths in plague of demons. both
: oozes and ghouls seem to regenerate on feel the power, with ghouls being
: somewhat more common.
On Feel the Power initially, Ghouls respawn at frequency 2 and Oozes respawn at frequency 6. Post-Demon, Ghouls (in the first monster slot) respawn at the higher frequency 15, and Oozes continue to respawn at frequency 6. So at the end of the game, you'd expect to see Ghouls more often, and you can still encounter any monster that normally respawns.
Ron said the Labyrinth is specifically excluded; Shocking Spheres respawn at frequency 32 no matter what stage of the game you're at. If it hadn't been excluded, the Labyrinth would actually be easier on the way out.
As far as surprises, the map format allows for monsters to only show up in the end game. For instance, They May Be Slow uses two of the three monster slots for Headless and Zombies. Bungie could have added, say, Oozes in monster slot 1, with an initial respawn frequency of 0 (never spawn), which would change to 15 at the end of the game. Bungie never took advantage of this, though: every monster in slot 1 is placed initially or has a chance of spawning before the frequency change.