Blizzard seems to have really enjoyed teasing hunters this expansion with the Dire Beast spell. Dire Beast is a talent which summons a random beast to fight for you for 15 seconds. Each zone in Pandaria has three different beasts it chooses from, and there are several instances where it will summon a beast which you're currently unable to tame. The most notable examples being the pterrorwings on Isle of Thunder and the gulp frogs on Timeless Isle.
There's also several beast families which have been in the game for a long time but remain untamable. Occasionally, these are added in a new expansion, such as the basilik and water strider in Mists of Pandaria. As a pet collector, I have high hopes that we'll see both old and new pet families added in Warlords of Draenor to fill our stables. Let's start with a family of beast so obvious that they already have a unique spell attributed to them in the game.