This is the picture of the mesh I wanted to UV unwrap. I've already re-meshed it, I just don't know where to add the seams to UV unwrap it because it has so much odd geometry.
The answer is you don't. Generally you want to retopologize the mesh by hand or addons and then bake the high poly version as textures onto the retopology version. If you where to UV map that mesh and then would go on to texturing you would not have a good time.
However lets say you still want to UV unwrap that high poly mesh. You have some ways of doing it.
U > Smart UV Unwrap
Or use Ctrl click and select edge loops at hidden locations like in the mouth and mark them as seem(Like in the mouth, behind the horn, at the end of the head and around the eyes and horns) It's hard to tell from the image how exactly the edges flow is but this could be quite hard with your mesh
Also if you choose to retopolgize, there are addons and websites however you can also use blender modifiers, like decimate or Remesh with Shrinkwrap for example.
If you want to know which way is the correct way the retopology way is the correct way, sometimes you will get away with static render meshes to not retopolegize them but you won't be able to progress as a 3d artist without knowing the proper ways.
Edit: I didn't know you retopologized it but I still stick by what I said, if you really can't work with this use a different retopolegy tools like: Instant remesh, retopo gun, retopo flow (although I don't recomend actually paying for it, total scam for 80$ but cracks exsist). Vanilla blender retopologizing is also quite good