From Guild Wars 2 Wiki
Mount licenses with random skin
- These licenses contain one random skin from their respective lists:
Mount licenses with selected skin
- These licenses contain one skin of your choice from their respective lists:
- All the mount skins can be dyed.
- You only can get the skins you don't own already i.e. it is not possible to choose/receive duplicate skins.
- You can get a skin for a mount you do not own, however, this will not unlock the respective mount. But once you have unlocked the mount, you can use the skin.