Callisto is one of the starter bosses. Callisto is a huge bear. There's always more than one Callisto in the area. So be on the look out!
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, you still need prayer "Soulsplit" (Level 54 prayer recommended) and your Strength Level at lest 50. You will need 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.