Yeah, just that... Maybe as a configurable option for those who prefer it not to be compatible...
Can I suggest something else? If so...
I would really like an option to spawn the horse away from the player in a safe block when using the Copper Horn, and make it run to the player from there, instead of directly spawning it in the player position.
Why?
That way it would seem it actually comes after hearing the horn instead of materializing on the spot.
...
Thank you for developing this mod, I had a blast with it.

You can add minecraft:camel to horseman:summonable entity tag, that should make it work with Copper Horns.

Spawning the horse further from a player is tricky, as it's not that easy to guarantee that horse will be able to reach a player (because of obstacles, etc)

I see... Thank you.

Yeah, it probably will need some work to get it right... I can help with testing and stuff if you decide to work on it.

So, I imagine I have to recompile the mod to make those changes, right? Is there a problem if I include a recompiled version of your mod in a modpack?

About the horse spawning, I imagine it's complex to do such thing... Hmm, Couldn't the game calculate pathfinding to the nearest reachable position of the player before spawning it?. I don't know how the game works, so maybe what I'm saying doesn't make sense

No, you need to create a datapack or use KubeJS. Recompiling is not needed.

Calculating a path for a horse could work, but I don't know how performant it would be, because I still have to find a valid spawn place for a horse in an area, and then check the path.

Update on this, I've added the minecraft:camel to the horseman:summonable entity tag list via a Datapack and it doesn't work (It just opens the inventory when trying to bind it)...
I'll try with kubeJS when I learn how to use it...

It looks like it doesn't work because Camels have custom logic when interacted and it blocks Copper Horn interactions.
I'll fix it in the next version.