Itīs pretty easy.
For example we give English Demi Lancers eastern armoured horse.
First we enter export_descr_unit.txt and find Demi Lancers.
Find there mount and change this mount : heavy horse to this mount: eastern armoured horse
Save it and close EDU.
Now we have problem because England doesnīt have eastern armoured horse.
So we must edit battle_models.modeldb (the best is if you download this edit
battle_models.modeldb file by KnightErrant)
Find this 28 mount_eastern_armoured_horse
Original
28 mount_eastern_armoured_horse
1.12 4
75 unit_models/Mounts/ME_Armoured_Horse/mount_eastern_armoured_horse_lod0.mesh 121
75 unit_models/Mounts/ME_Armoured_Horse/mount_eastern_armoured_horse_lod1.mesh 900
75 unit_models/Mounts/ME_Armoured_Horse/mount_eastern_armoured_horse_lod2.mesh 2500
75 unit_models/Mounts/ME_Armoured_Horse/mount_eastern_armoured_horse_lod3.mesh 10000
9
6 russia
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_russia.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
59 unit_sprites/russia_Mount_Eastern_Armoured_Horse_sprite.spr
9 byzantium
81 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_byzantium.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
62 unit_sprites/byzantium_Mount_Eastern_Armoured_Horse_sprite.spr
5 moors
77 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_moors.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
58 unit_sprites/moors_Mount_Eastern_Armoured_Horse_sprite.spr
5 egypt
77 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_egypt.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
58 unit_sprites/egypt_Mount_Eastern_Armoured_Horse_sprite.spr
5 turks
77 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_turks.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
58 unit_sprites/turks_Mount_Eastern_Armoured_Horse_sprite.spr
7 mongols
79 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_mongols.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
60 unit_sprites/mongols_Mount_Eastern_Armoured_Horse_sprite.spr
8 timurids
80 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_timurids.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
61 unit_sprites/timurids_Mount_Eastern_Armoured_Horse_sprite.spr
5 slave
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_rebels.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
58 unit_sprites/slave_Mount_Eastern_Armoured_Horse_sprite.spr
4 merc
77 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_mercs.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
57 unit_sprites/merc_Mount_Eastern_Armoured_Horse_sprite.spr 0 1
5 Horse
14 fs_heavy_horse 0
0 0 -1 0 0 0 0 0 0
Edited
28 mount_eastern_armoured_horse
1.12 4
75 unit_models/Mounts/ME_Armoured_Horse/mount_eastern_armoured_horse_lod0.mesh 121
75 unit_models/Mounts/ME_Armoured_Horse/mount_eastern_armoured_horse_lod1.mesh 900
75 unit_models/Mounts/ME_Armoured_Horse/mount_eastern_armoured_horse_lod2.mesh 2500
75 unit_models/Mounts/ME_Armoured_Horse/mount_eastern_armoured_horse_lod3.mesh 10000
10
6 russia
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_russia.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
59 unit_sprites/russia_Mount_Eastern_Armoured_Horse_sprite.spr
9 byzantium
81 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_byzantium.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
62 unit_sprites/byzantium_Mount_Eastern_Armoured_Horse_sprite.spr
5 moors
77 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_moors.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
58 unit_sprites/moors_Mount_Eastern_Armoured_Horse_sprite.spr
5 egypt
77 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_egypt.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
58 unit_sprites/egypt_Mount_Eastern_Armoured_Horse_sprite.spr
5 turks
77 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_turks.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
58 unit_sprites/turks_Mount_Eastern_Armoured_Horse_sprite.spr
7 mongols
79 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_mongols.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
60 unit_sprites/mongols_Mount_Eastern_Armoured_Horse_sprite.spr
7 england
79 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_england.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
60 unit_sprites/mongols_Mount_Eastern_Armoured_Horse_sprite.spr
8 timurids
80 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_timurids.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
61 unit_sprites/timurids_Mount_Eastern_Armoured_Horse_sprite.spr
5 slave
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_rebels.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
58 unit_sprites/slave_Mount_Eastern_Armoured_Horse_sprite.spr
4 merc
77 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_mercs.texture
78 unit_models/Mounts/ME_Armoured_Horse/textures/me_armoured_horse_normal.texture
57 unit_sprites/merc_Mount_Eastern_Armoured_Horse_sprite.spr 0 1
5 Horse
14 fs_heavy_horse 0
0 0 -1 0 0 0 0 0 0
Save ti and close.
Last part is adding texture to England horse.
Just find egypt eastern armoured horse (me_armoured_horse_egypt.texture) and recolorize it in Photoshop and save it with name me_armoured_horse_england.texture here data\unit_models\mounts\me_armoured_horse\textures
Result
If you adding horse what that nation already have (for example mailed horse to england bodyguard (mailed horse have england templars)) everything what you must edit is edit EDU.
I hope it help you a bit.