Someone say stats?
Kangacorns: +3 MOV, JMP = 1
Normal kangacorn: +50% STR, +10% SPD... -30% EVA
Armored Kangacorn: +50% STR, +30% PDF... -30% EVA
Useable by 1h sword, spear and missile weapon users.
Owls: +2 MOV, JMP = 4 (dunno if that has any effect for the owls)
Red/Blue Giant Owl: +30% STR, +30% EVA, +30% SPD... -30% PDF
White Giant Owl (Armored): +30% STR, +30% EVA... -10% PDF
Useable by 1h sword, spear, and most archers (not Seneca). Archers have their range increased by 1 while on an owl.
Bonuses from equipment that affect attributes will also be multiplied. It always rounds the final stat down.
All that the game says is that "defensive skills" can't be used. I'm pretty sure that includes Parry, Shield Defense and Guard, possibly Armor Defense and Deflect Missiles as well. I know Dodge and Mind's Eye work, though.
There's one other small drawback: whenever you mount/dismount, your next turn will be slightly delayed. It's just one of many things that affects turn order though, and the extra speed from some mounts can balance it out (or maybe even improve your delay).