Scary Jack

Scary Jack is a halloween-themed trinket that summons a pet.

Source
The item can be purchased in East End Police Station (heroes) or The Gotham Tap Room (villains) during The Witching Hour event.

Scary Jack
Upon activating the trinket, Scary Jack is summoned to aid the player in battle. Scary Jack taunts enemies that are attacking you.

Attacks

 * Ball - does a single attack for 40 damage (on training dummy).
 * Beam - does four consecutive attacks, each for 14 damage (on training dummy).