A specifically large orca, she will spawn in the southeast corner of Howling Fjord when somebody takes the Alliance flight from Valgarde to the Explorers' League Outpost. She is generally thought by players to be scenery to look at on the way — much like Shatterspear Village and the Dun Morogh airfield — because, although she is shown as aggressive, she will not attack you and cannot be attacked. The other reason is Big Shirl's extraordinarily large HP.
When the player taking the flight starts to pass the sea, Big Shirl will follow him/her along the way. She will despawn shortly as the player flies back towards the land.