I just started thinking about the beasts the other day and so far i have this:
Air avatar: flying bison, you cant argue that one

Fire avatar: that dragon think Roku rides, once again aang called it roku's spirit beast so i'd say this one is fact to (can anyone tell me what the sun spirit's physical form is)

Earth avatar: the badger moles, i'll get to the thinking that led me to this.

Water avatar: no clue, but i'd guess some kind of seacreature, maybe a giant coy.

Following the theory that airbenders learned from flying bison, it would make since that their are more badger moles, in the world (remember all the bison in one of aangs flashbacks?) So i'd say that the earth benders ride the badger moles and travel through the earth. Of course, im not really sure. Also, once again i'd like to know what physical form the sun spirit takes in asian legends(particualarly chinese ones)

He called it an animal guide. And we saw some guy riding a bagdermole in the Blind Bandit so there are probably enough to go around.

and the earthbenders learned??earthbending from the badger moles.... and the waterbenders hmm who did they lear from...??the moon spirit??? and the firebenders... the sun... spirit???

Earth= Badger moles, Air = bison, fire = sun or dragon (we'll see in season 3) Water = Moon. We will know all of this soon. In SOTN Yue explained, in COTL, the scripture explained, we'll learn the others soon.

are u talking about learning or the beasts, cause i dont think the firenation ride the sun. . Seriously though, im just trying to see if anyone else has ideas about the avatar's spiriy beasts.

oh, it's like bison, dragon, badger mole or ostrich horse, and those goat things. I think, we'll learn I'm sure