I think you need to look for the various horse entries in descr_model_battle.txt
ie.
type horse_light
skeleton fs_fast_horse
scale 0.875
indiv_range 40
texture romans_julii, data/models_unit/textures/horse_light_julii.tga
......
texture thrace, data/models_unit/textures/horse_light_thrace.tga
model_flexi data/models_unit/mount_horse_light_400.CAS, 15
model_flexi data/models_unit/mount_horse_light_300.CAS, 30
model_flexi data/models_unit/mount_horse_light_200.CAS, 60
model_flexi data/models_unit/mount_horse_light_100.CAS, 80
model_flexi data/models_unit/mount_horse_light_70.CAS, max
model_sprite 100.0, data/sprites/horse_light_sprite.spr
model_sprite slave, 100.0, data/sprites/slave_horse_light_sprite.spr
model_sprite thrace, 100.0, data/sprites/thrace_horse_light_sprite.spr......
Compare that with the same section from the mod you want to import the horses from - copy the texture, .cas & sprite files over from that other mod - and either re-name those files to suit the text file or ammend the text file to reference the new file names if different.