Zombie horses spawn too often in Superflat worlds in Peaceful difficulty

HUNDREDS of Zombie Horses spawn in Superflat Creative World on Peaceful then burn with sunrise.

There are TOO MANY zombie horses spawning in naturally on a Creative Superflat world set to /difficulty peaceful, that then die when the sun rises. We can no longer work in our flat worlds in creative as the burning and spawning is too disruptive.

• Steps to reproduce the issue
  Launch and create a SUPERFLAT CREATIVE WORLD on the latest snapshot or pre-release in either multiplayer or singleplayer. Set to night. Hundreds of Zombie horses spawn in naturally disrupting gameplay and building activities with my students.
  

• Expected result
  While they may not be considered a hostile mob, we expect much FEWER of the zombie horses to spawn, IF AT ALL. They should only spawn in in Survival mode, and with zombies mounted on them.
  

• Actual result
  Zombie horses, altho passive mobs, should not spawn-in in these high numbers. There are many many more of these horses than regular horses, chickens, sheep or cows.