In Java Edition of Minecraft 1.12.2, if you use a console command to have a Vex as a passenger to most mobs, the mob's AI will essentially break. The mob will walk around for a short duration, in seldom cases causing their head to go backwards 180 degrees, and then do one of the following if it cannot find an attack target (if it is hostile):

-Become stationary and spin endlessly until attacked
-Become stationary and hop spin 180 degrees endlessly until attacked
-Become stationary and spin as well as flail arms endlessly until attacked (Happens most with Skeletons and Wither Skeletons)
-Become stationary and hop spin 180 degrees with right arm raised endlessly until attacked (Vindicators only)
-Will Hop around erratically in a triangular pattern (Rabbits only)
-Will fly around slowly in a circle (Parrots only)

Vex, Spiders, Slimes, and Magma Cubes are unaffected by this glitch and behave as normal. The Wither will ascend endlessly erratically firing Wither Skulls until it can hit itself to kill the Vex inside, in which it will slowly float back down. Iron Golems and sometimes Wolves will be able to hit and kill the Vex that is riding it.

This is odd as other mobs behave usually normal if a Vex is around but not a passenger.

Video Proof with different mobs: https://www.youtube.com/watch?v=mve1yGm43lI
Video Proof with Wither Skeletons (I also gave them blaze rods for humor): https://www.youtube.com/watch?v=mf0UuUp-cN0