The mod has only one .esp file and I deleted it manually. When I saw it didn't work, I installed the mod again with NMM and uninstalled it (again with NMM). But I still have the saddle texture from the mod. So I deleted all files in the horse textures directory, it was empty. The horses did go back to the vanilla texture, but not the saddles...
EDIT : I even tried to delete the horse directory in both textures and meshes, just out of curiosity and lauch my save. The horses look like shit, but the saddle textures are still there!
you can always completely uninstall and reinstall the game. when modding skyrim i like to keep a "clean" copy for this reason. not all mod authors know what they're doing.