The mod Realistic Horse Genetics replaces horses with it's own custom horses, but Astikor carts cannot be attached to them.

Could compat for this be added?

Our configuration file is located at "config/astikorcarts-common.toml" You can add any entities to the "pullEntities" list. The mod page you linked's FAQ conveniently explains what is needed to have their horses be recognized by our mod.

Oh, I didn't realize that, thanks!

Happy carting!