StreamOfTheSky
Adventurer
Halivar said Giant Eagle, which is from the MM and oddly enough, a magical beast and not an animal. Dire Eagle is an animal and from RoS. Completely different things. 

...They made Chocobos for D&D!? I see your Avatar, brings me back to my FFXI days. Sigh.I know it may not fall within range of approved material, but I want to suggest it anyway.
How about the Black Chocobo from Dragon Magazine 323 pg 35?
It's a bit low on the carrying capacity due to being bipedal...
...but it is a 4 HD animal with a 60 ft land speed and a fly speed of 90 ft. (good).
Less dexterity, constitution and natural armor than your guard bat idea however.
Makes me wonder what a dire black chocobo would be like.