Many units in the game have special skins available to them. If you have multiple skins for a unit you can select which one to use on the unit screen. Note that if the skin is provided by a memento, you must own the memento but don't have to be wearing it to use it.
To see if there are alternative skins available for a unit, you can check the unit page for that unit.
Pages in category "Skins"
The following 133 pages are in this category, out of 133 total.