What is a Tigon you ask? Tigon are a very ancient race, and therefore not much is known about them. So little, in fact, that the name Tigon isn't official, but rather the name given to models using the Tigon body from datamining. Roar from Karazhan being the most famous of "Tigons" and High priestess Kilnara from Zul'Gurub transforms into a 'Black Tigon'. They existed far before playable Worgen (They do not use the Worgen animations or structure AFAIK) and have never had a proper reveal by Blizzard. Maybe if we amass tweets to Mr. Kosak he'll spill the beans on just what they are.
Let your speculations run wild. What are they? Where do they come from? Are they loa? an entirely new race? Natives of Azeroth on some faraway continent?