Try the Commands Troubleshooting and Help page if you get stuff with server errors.
Minecraft Zombie Horse NBT Data Tags
Using the summon command to spawn in a minecraft zombie horse is simple, take the command and enter into chat and presto you have a zombie horse.
How to summon a tame Zombie Horse with saddle
The summon command has tags for summoning a tame zombie horse with saddle. Tame allows you to ride the horse, you can use the Owner UUID (players special id given by Mojang) to make it belong to a user. The saddleitem allows you to place an item in that slot for the horse (it's just saddles so far).
/summon minecraft:zombie_horse ~ ~ ~ {SaddleItem:{id:saddle,Count:1},Tame:1}How to summon a Zombie Horse wearing armor
Unfortunately Zombie Horse can not wear armor
How to summon a baby zombie horse (foal)
Like most passive mobs in Minecraft, you can summon a foal by setting the Age tag to a negative number (under the "Additional" tab).
/summon minecraft:zombie_horse ~ ~ ~ {Age:-1200} (Grows up in 60 seconds)/summon minecraft:zombie_horse ~ ~ ~ {Age:-6000} (Grows up in 5 minutes)
Summon zombie horse owned by player, Owner NBT Data Tag (1.16+)
You can set which player owns the zombie_horse by setting the UUID, which is now a int-array.
/summon minecraft:zombie_horse ~ ~ ~ [I;-132296786,2112623056,-1486552928,-920753162]Summon zombie horse owned by player, OwnerUUID NBT Data Tag (before 1.16)
You can set which player owns the zombie_horse by setting the UUID.
/summon minecraft:zombie_horse ~ ~ ~ {OwnerUUID:00000000-0000-0000-0000-000000000000}Suggestions or found a bug
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If you're looking for a particular summon mob generator you can link to these specific versions of the command generator.