Item Thief is a type of enemy that appears in Adventure Mode in Hyrule Warriors and Hyrule Warriors Legends. As their name implies, they steal items from the player. If they steal an item they will teleport away and then appear in another location. The player will be unable to use the stolen item until the Item Thief is defeated. In some cases, they can be ignored if the player does not need (or can get by) without that item within the current scenario, however in other cases it is wise to kill them to regain said item (for example, the player may be required to retrieve their stolen Bombs to weaken a King Dodongo).
It should be noted that the Item Thief are a type of enemy unit, that can appear as various Enforcer class enemies (such as Lizalfos, Dinolfos, Gibdos, etc). The only difference is their name and unique behavior.