Goto a random wildhorse, place a crafted saddle on them. Right click and hold space.
You can see the bug in action here: YouTube

Modpack: "One Life" - Popular SMP
Server is running SpongeForge; although this bug also occurs in singleplayer.
Modpack is running MoCreatures V 12.0.5
Modpack is running CustomMobSpawner V 3.11.4