Forsaken hunters automatically know how to tame undead creatures. Hunters of other races must learn the skill from a Simple Tome of Bone-Binding which is a rare drop from miscellaneous Elite NPCs throughout Maldraxxus and its associated dungeons: Plaguefall and the Theater of Pain.
Retains its original name after taming. Ohgan is part of the Bloodlord Mandokir encounter and initially serves as the boss's mount. To tame him you'll have to wait until Mandokir dismounts and Ohgan kills all of the Chained Spirits that surround the arena; before then he refuses to be distracted by you.
Level scaling: Most NPCs will scale with the Hunter's level, within the constraints of their level range. Hunters can tame regular NPCs up to 2 levels higher than them, but can only tame elite NPCs of the hunter's level or below.