The Deer mount is a rather simple but elegant model.
I believe it came to be with the Irene update, as it suits the environment there well.
Funny on F2P, the Deer is thrown together with the Moose mounts. On P2P, they are seperate.
Skins
Forest Knight
This skin got available on launch with Prophetic cards "Lost Relics".
It now drops for P2P from obs and nihaz raid bosses.
For F2P it's available from the Free Trader's tickets.