Mimiron's Head is a 310% speed flying mount.
This item drops from Yogg-Saron in the 25-man version of Ulduar. It drops only if none of the Watchers are helping.
Only one drops per kill.
Technically, it is the slightly modified head of V0-L7R-0N, not Mimiron. Though it may be a Head belonging to Mimiron and not the Head of Mimiron.
Obtaining this mount awards the Feat of Strength, And I'll Form the Head!.