Source: Xanathar's Guide to Everything

Wondrous item, common

This hat has 3 charges. While holding the hat, you can use an action to expend 1 of its charges and speak a command word that summons your choice of a bat, a frog, or a rat (see the Player's Handbook or the Monster Manual for statistics). The summoned creature magically appears in the hat and tries to get away from you as quickly as possible. The creature is neither friendly nor hostile, and it isn't under your control. It behaves as an ordinary creature of its kind and disappears after 1 hour or when it drops to 0 hit points. The hat regains all expended charges daily at dawn.