When creating a new zombie or skeleton horse, or donkey or mule, and using
/get data entity to view it’s speed, it is not random.
For Skeleton horses & Zombie horses, the minecraft:movement_speed is: 0.20000000298023224d
For Donkeys & Mules, the minecraft:movement_speed is: 0.17499999701976776d
It also does not display the Donkey/Mule minecraft:jump_strength.
When spawning a baby donkey or mule (by clicking a spawn egg on the donkey), the minecraft:movement_speed and minecraft:jump_strength do change and are randomly set.
This behaviour does not occur when using the Horse spawn egg.
Environment
Create a new survival world with Commands enabled
Get a Zombie horse/ Skeleton horse/ Donkey/ Mule spawn egg
Use the spawn egg and use /data get entity (UUID) when looking at the mob
Observe the speed and jump height not being random.
Skeleton horses & Zombie horses also spawn naturally with set speeds only randomising jump strength and Skeleton horses. Donkey’s and Mules only randomise health. This is an intended behaviour.