Pack Mule

Pack Mules can be found at Odesyus' Landing on Azuremyst Isle and in the Eastern Plaguelands.

See Mule.

Quests
They are involved in the Eastern Plaguelands quest Quest:Writ of Safe Passage.

Speculation
This is rumored to be released in future patches, there is NO conformation though.

"Pack Mule — Summons a pack mule, giving you access to any bank items for 3 min. The mule may only be summoned once. Summon Faithful Mule — Summons your faithful pack mule, giving you access to any bank items for 3 min. "