Put simply, a Chocobo is a large bird. It is unclear whether they're a single species or a genus, as there are quite a few types. Based on the available data, though, it is clear that Chocobos:
- Come in various colors
- May or may not be able to fly or swim. This is dependent on type; see the link above.
- Are large enough to be ridden, and strong enough to be ridden by armored knights
- Are intelligent enough to be tamed and trained
- Typically reside in grasslands and forests
- Are herbivorous
- Are fast enough that racing them is a sport
- Are able to pull carts
- Have appreciable offensive capabilities
- Lay eggs
Given the above, how might Chocobos evolve?
Note: Because Final Fantasy is a RPG, Chocobos are stated in source works to have magic, be synergists, etc. Answers may either explain this or completely ignore it at the answerer's discretion.
A list of all of the Anatomically Correct questions can be found here