Try the Commands Troubleshooting and Help page if you get stuff with server errors.
Minecraft Zombie NBT Data Tags
A few tags for Zombie Zombies. The CanBreakDoors:1 doors is useful for map makers.
Summon a baby zombie
Are they cute or a little tragic? There's a special tag for a baby zombie, unlike passive mobs they never grow up.
/summon minecraft:zombie ~ ~ ~ {IsBaby:1}Summon a baby zombie riding a chicken
Baby zombies have an additional 5% chance of spawning as a chicken jockey in Java. But here is the summon chicken jocky command if you want to make it happen. Note that the chicken has the nbt tag IsChickenJockey which allows despawn as per usual hostile mobs.
/summon minecraft:chicken ~ ~ ~ {IsChickenJockey:1,Passengers:[{id:zombie,IsBaby:1}]}DrownedConversionTime and InWaterTime NBT Data
If a zombie is in water for 30 seconds (600 ticks) it begins converting into a drowned. After 15 seconds (300 ticks) the zombie becomes a drowned. Both values have a default of -1, which means no conversion is happening.
What about Zombie Villagers
Try the full villager mob generator, which has a easy select of types, professions and trades
What about Drowned, and Husks
Drowned use a different id "drowned", click here for drowned
/summon minecraft:drowned ~ ~ ~
Husks use a different id "husk", click here for husks
/summon minecraft:husk ~ ~ ~
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.