Dalamar
Adventurer
Others
Make a check when you've grown one? Get real
The MM isn't exactly plentiful with this kind of information, but I'm going to guess from the way all the dire animals are depicted as feral, angry and dangerous, that they're carnivorous. So your dire bat eats anything small and living: fish, small birds, rats, normal bats, very big bugs or lots and lots of smaller ones. Or you could feed him rations, though he'll need two to be fully satiated.
"Well... I do have enough, for six horses and a mule for that time... but it'll mean I'd need to sell you some of what I'm holding for my own mules, and if my supplies will be late, they might starve. So it'll cost you more, so I can pay some speed extra for my supplier."
Make a check when you've grown one? Get real

The MM isn't exactly plentiful with this kind of information, but I'm going to guess from the way all the dire animals are depicted as feral, angry and dangerous, that they're carnivorous. So your dire bat eats anything small and living: fish, small birds, rats, normal bats, very big bugs or lots and lots of smaller ones. Or you could feed him rations, though he'll need two to be fully satiated.
"Well... I do have enough, for six horses and a mule for that time... but it'll mean I'd need to sell you some of what I'm holding for my own mules, and if my supplies will be late, they might starve. So it'll cost you more, so I can pay some speed extra for my supplier."