Foxhounds significantly reduce spawn rate for nether mobs. Very frustrating when trying to catch ghasts for Extra Utilities Angel Ring or finding Wither skeletons. I performed a test by disabling Foxhounds and creating a new world. Spawn rate returned to normal.

Quark-r1.6-177.jar
AutoRegLib-1.3-32.jar
forge-14.23.5.2846

Same versions, same observation.
Took me a while to figure out why my nether fortresses were completely empty of all mobs. (never even saw a ghast)
So in desperation I used /cofh killall and it reported 123 foxhounds killed...
I've since reduced the spawn "weight" from 10 to 2.

Edit: Uppon further testing I still regularly got 30+ foxhounds in a killall, so I ended up disabling them altogether.

This is caused by EntityFoxhound extending EntityWolf (which is an animal and doesn't despawn), but being registered for spawning as a hostile mob. Eventually, your entire mob cap in the nether will be full of foxhounds, completely blocking the spawning of other hostile mobs.