2.0 had ponies, 3.0 had birds so what will 4.0 have?

[Dualblade]

Landsharks!

[Daenerys_Sedai]

seahorses, rays, and dolphins would be my guess.

[Felis]

Flying sharks, a Tornado Shark
http://xivdb.com/item/12798/tornado+shark
He can swim, he can fly, and he can float on land (see Void Arc)

[ChakoTako]

Sea serpents

Gold Leviathan but not really Leviathan

Yes! Like the Chinese dragons in mythology or like the dragon symbol in ' The Wheel of Time' . In sea or in in the sky, riding that would be amazing

[Vexander]

Turtles is my guess, to compliment the swimming and diving aspect of gameplay.

[Zohar_Lahar]

Manta Rays

[AlexanderThorolund]

Im here for the whale suggestion. Whales would be very cool, theyre such large creatures theres loads you could do with them aesthetically

[Zojha]

Im here for the whale suggestion. Whales would be very cool, theyre such large creatures theres loads you could do with them aesthetically

I think whales would be better for the cash shop.

Kudos to who gets the pun.

[ErikMynhier]

I'm going to stab in the dark and say Snakes (which will lead to the wither the blue Dragon or the Turtle & Snake) or cats that lead to a white Tiger.

[Niwashi]

Snakes or sea serpents and turtles fit a water theme better than cats do, so I think it's going to be Genbu's turn. (But then we should get cats for 5.0.)

As for getting aquatic mounts (no matter what type they pick) to take to the sky, the whole notion of skyfishing has already established that numerous creatures that (if they existed) could only be found in the oceans on Earth, have taken to flying the skies of Hydaelyn. All we need is ones that are large enough to ride and can travel between both.