Trainer Troops are special Troops that can only be used to level up other Troops. They cannot be levelled up or used in battles, but will grant more experience than other Troops of equal rarity when used in training.
As with Trainer Heroes, Trainer Troops of higher rarity grant more experience.
Players have a chance to obtain Trainer Troops from Titan Loot, War Chests, and Mystic Vision. You can view them you own in the Inventory (under the "Troops" tab) or in the Barracks.