I was trying to make a wither skeleton riding an armored horse.

When I spawned it in using commands, it was slightly flying above where it should be sitting.

Use this command to summon it

/summon Horse ~ ~ ~ {Passengers:[{id:"Wither_Skeleton"}]}

Affect passengers:

1. chicken
2. cow
3. donkey
4. enderman
5. endermite
6. horse
7. husk
8. llama
9. mooshroom
10. mule
11. ocelot
12. pig
13. polar bear
14. rabbit
15. sheep
16. silverfish
17. skeleton
18. skeleton horse
19. stray
20. vex
21. wither skeleton
22. wolf
23. zombie
24. zombie horse
25. zombie pigman
26. zombie villager