Slime/MagmaCube can't ride a horse very well.

to reproduce:

1. run it:

   /summon minecraft:horse ~ ~ ~ {Passengers:[{id:"minecraft:magma_cube"}],Tame:true}

1. create a iron golem near the horseman.
2. the rider and the horse turn towards to the iron golem, but they still walk by the same direction. (always go straght)