Riding on birds is not awesome enough. Let's ride Ferus instead.
Rune Knight mount revealed: armored Green Ferus.
Source + picture:
here
Other possible information but no sources:
1) Rune Knights are able to mount and ride a Green Ferus. However, mounted Rune Knights can only use spear type weapons, and are unable to use skills such as Bowling Bash (only spear exclusive skills). Skills that use 'Rune', a type of magic power available to Rune Knights are exclusive to Two-Hand Sword builds.
2) Royal Guards will be able to equip two-handed shields, and have exclusive defensive/offensive skills to use with them.
3) Rangers will be able to use either a Falcon or Wolf, though not both at the same time. They have unique skills available depending on which companion they have.
4) Mechanics appear to be able to equip a special type of armor, similar to the 'Magitech Armor' from Final Fantasy 6. However this powerful armor requires the mechanic to de-equip his cart, so while they can use exlusive skills with the armor, they can no longer use cart exclusive skills.
Taken from: a post in RMS, but no source was given