Callisto is one of the starter bosses. Callisto is a huge bear. There's always more than one Callisto in the area, but they are not aggresive.
You can find Callisto in the desert by using the Boss Teleport System or in Sponsor Zone, also known as Ownercape or Ownercape Zone (you must be sponsor).
Even though Callisto is a starter boss, Its recommended to have your Strength Level at least 50, and a fast hitting weapon, such as a Glock, Assault Rifle, or an Icy katana. You will also need armor, such as Torva, Flame Pernix, Dragonbone Melee Armor or higher/better armor.