Being mythic is awesome!
The rules from Pathfinder Roleplaying Game: Mythic Adventures provide many options for upgrading a character to the realm of mythic heroes. However, while there are numerous mythic spells and a long list of mythic feats (and even more are available in Mythic Options: The Missing Core Feats, which gives a mythic version of every feat from the core rules that does not already have one), none of these options are specifically designed to work with the dragonrider class (from The Genius Guide to the Dragonrider).
There are champion and guardian path abilities that tie into some dragornider character concepts, but nothing that can add a mythic touch to the dragonrider's core class features, or to a dragon steed.
Mythic Options: Mythic Dragonrider Class Features offers mythic upgrades for every rogue class feature and talent from the core rulebook. It allows dragonrider characters to focus their mythic power entirely on their unique class features, or select just a few such abilities to upgrade while adding the while adding the normal options of mythic feats and path abilities.