Information

Minecraft version: 1.20.01
Modloader: Forge

Mod name: Zombie Horse Spawn

Feature description

Allow zombie horses to be rideable.
Only one per time is being allowed and when restarted the server, all zombies horses went back to not rideable and the saddle disappears, so modifying the game to allow it naturally and for all zombie horses would fix it.

I believe that you can try to use this mod as base
https://modrinth.com/mod/rideable-zombie-horses