The goal:

Exporting Millhorse with morphs and animations from Poser 12 to Blender 3.4.
In Blender I will add hair as mane and tail on the horse.
The final images/animations will be rendered in Blender and used in a game project.

What I have tried so far:

Exporting from Poser as fbx which resulted in loosing all morphs.

Exporting from Poser as Alembic (I read about that somewhere) which worked keeping morphs and animations. BUT when trying to add hair Blender gives error: "Invalid UV map... islands overlapping". I know what it means but I can not for my life figure out how to fix it.

Attached image of morphed horse with material rendered in Blender.

Any suggestions, links and hints would be much appreciated.

select your hair mesh object, switch to Edit mode, and in the UV Editor, make sure all islands are not overlapping and have a bit of space between each other:

I hope this helps 

Yes, I know about that one but is it not possible to add hair directly on the horse mesh? 
For example if I am to create and use a separate mesh for the horse mane how will that follow the animations? The horse neck has 3 bones+ the head.

If possible I would prefer to add hair straight on to the horse mesh. But....
The problem I am having is the existing horse UV map/ texture, imported and added on the horse, will not show up in UV editor. 
Does that mean I have to create a new UV map in Blender including all the "cutting of the mesh" and such? Seem like waste of time since I have 24 different horses and breed already made which only need mane and tail in Blender. :)

More suggestions please. :)

If I understand correctly, Blender isn't even letting you work on the UV map of the horse at all. You might try a stand-alone UV-map program or, just an idea: you might try exporting the horse figure as OBJ, just for the UV map.

More than one way to skin a cat.

I just looked for Millhorse and it is originally a Daz product back from maybe V2 to V4 release so it's obj is in a much older format and all the UV are overlapping if I remember right those UV from OBJ back then were imported into UVMapper classic and the UV needed to be reorganized usually by groups so they were not over lapping even Victoria 2 thru V4 you had to repack the UVs if you wanted to create custom textures you can still get UVMapper Classic for free and he still has the tutorials up on his site on how to recreate the UV for use in other programs or to use to make custom textures  I actually still have UVMapper Classic archived on one of my backs ups for use with older Models   http://www.uvmapper.com/downloads.html 

I hope this helps 

Aha, I see. Makes sense. 
I have not worked much with UV mapping so I simply thought the texture was it and it did not look as if anything was overlapping. See attached pic.

I will test making a "mane object" for each neck part, put hair on them and have them follow the animations. Should work right? :)

This is only part of the texture since one is not allowed to "redistribute" it. So this is not the UV map? If it is, could the pixels touching in some areas be considered overlapping?

Sorry for all the beginner questions. I do appreciate everyones input and guidance a lot. 😊

That is a texture map based off the UV as to the question of animation the texture maps should not affect the animations and any potential issue with animation can be another issues all together and never worry about asking questions all the members in this forum if they can help they will speak up and give advice based on what they know and we will do our best to try and help. 

Posting this in case someone else has same issue. 

I was able to get it done following THIS tutorial and adding the horse texture after the hair was added. 
No need to create new UV map and no errors. Am soooooo happy. 😊

Big thank you to everyone supporting and helping. Hugs.

Pic showing first, rough try for a dynamic mane created directly on the horse mesh. Horse with animations imported from Poser as Alembic file which kept all morphs.

Can I change the title and add a "Solved" somehow? 😊

No need to mark it solved if others are having the same issue or come across it in the future they will be able to look thru the thread and see that a resolution was reached plus sometimes editing the title might cause some of the post in the thread to disappear. I am really glad you were able to find a solution with the help of the community :thumbsup: 

Ok, good. Thank you. 😊
I'm sure I will be back with other questions so it is really good to know there is a friendly and helpful community. 

We have a great friendly Blender community here that is one thing I can truly attest to look forward to see your future post good luck