The whale had been rumored to appear in the process of an underwater quest that took place in the Undersea Temple.
The creature is a giant whale-like white monster, with its head wide enough to allow at least seven players to ride it while standing horizontally on the whale.
The whale can be used as a method of transporting players from Undersea Temple to Alfheim.
Yui had a wish of seeing and riding a whale, so Kirito and his friends tried to fulfill that wish by initiating the Deep Sea Plunderers quest. After completing the quest, Leviathan the Sea Lord summoned a giant whale to send them back to Alfheim.