Blizzard has updated the 500 Mounts Meta Achievement mount, it is now the Otterwordly Ottuk Carrier, an armored Otter mount with Soulshape-looking effects. With this, the mount also flies now!
Otterworldly Ottuk Carrier Ottuk
Description: Some ottuks get into the spirit of carrying people around more than others.
Source: Achievement: Thanks for the Carry!
Model id: 102081
The mount also now flies and swims fast!