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.
This unique beast first appeared in the Blood Elf Heritage quest chain (blood elf-only), but since Patch 11.2.7 other races can encounter it by running "The Children of Blood" chapter of the "Lorewalking: The Elves of Quel'thalas" campaign, given by Lorewalker Cho in Stormwind, Orgrimmar, and Dornogal. See Wowhead's Lorewalking Overview for more information.
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.