http://www.wowhead.com/?spell=5784#comments
By
knottyguy on 02/04/2009 (Patch 3.0.8)It's a spell learned from your Warlock trainer at level 30. To find the spell, open your character screen, click on the "Pets" tab, then "Mount" tab. The horseshoe icon is your spell.
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By
Erasto 15 days ago (Patch 3.2.0)Do not that it is patch 3.2 now so they've reduced the cap even lower down to 20!