When you have a Lame Horse/Warhorse, you can go to the party menu and upgrade a troop, which needs a horse/warhorse.
When using the lame horse for that purpose, you can then cancel the upgrade by closing the menu and voilà, you have a regular horse/warhorse instead of the lame one.
When using the lame horse for that purpose, you can then cancel the upgrade by closing the menu and voilà, you have a regular horse/warhorse instead of the lame one.