At level 50, Mir is large enough for Evan to ride him. The cost of the saddle is 10,000,000 mesos.
Mir grows yet again at level 80, and needs to have a larger mount. It costs 30,000,000 mesos.
The last mount is available at level 120. By then, Mir is able to fly with Evan on his back, but only after the Chaos Patch. The mount costs 60,000,000 mesos.