Cu Sidhe. Because they like to kill stuff. And can be built using all the magics. So you can have many variations.
Hiryu. It comes in a great shade of green, hue 34140. If Cu Sidhre came in the same hue I would choose that over a blaze Cu in a heart beat.
giant beetle because you can carry loot in it. (I still wish they had costumes... a lady bug would be awesome) It is a must have for a tamer t-hunter and for looting ships on the seas.
A Nightmare named Nova. I have had her since first building up enough bard and taming skills over 15 years ago. She must have killed my character Cricket a handful of times before I finally got her. One of my best memories of the game. She is all trained up now and is a beast. As for my mage Salem and my sampire Penny de Clown, they ride llamas. Just something cute about them rampaging through a champ spawn on those unassuming mounts. My Paladin archer John Carter rides a Charger of the Fallen. My craftsman Ghost of Gramps rides a beetle. My imbuer, alchemist, and beggar Uncle Buck and my lumberjack and fletcher Kiblet just hoof it. On my my other account my thieves Buckwheat and Billy Elliot go without mounts as well.