These creatures are a mix of a marsupial and a bird. It has a stronger frame than birds on earth, and are very large. These birds are inspired by the extinct moa. They are hunted by thunderbirds and Griffins, and run along the ground. Females carry their eggs in pouches on their sides, along with food. Males also have pouches, but they only carry food. Males can carry about 100 lbs., while females can carry twice that much. Their wings are used primarily to cover these pouches when full, so as to prevent spoiling of the food inside. Males and females bond for life, and these birds normally travel in small herds of 3-6.
(I want the females to be able to carry an average human male.) With their large size, and potential carrying capacity, would these creatures be suitable for use by humans? Would the fact that they're birds be a deterrent?
For all of those who are interested here is some history of my world. There is no evolution of mythological creatures, although they have adapted slightly to survive.
A long time ago, the sun began to grow distant. The people of the time feared for the safety of their world, so they convinced the great wizards to undergo the great reformation. The planet was reformed using magic, and it could now sustain life indefinitely. However, all the Wizards perished due t the great duress they underwent. They were also not able to protect all the creatures, so some got thrown together, and many new creatures and plants were created. Hence the existence of unicorns, griffins, dragons, etc.