One (joking?) explanation someone on Reddit said was that when Hinaazu evolves into Doduo, the third head gets eaten by the other two. And when Doduo evolves into Dodrio, the third head comes back as the angry head.

Blue_Stuff said:

One (joking?) explanation someone on Reddit said was that when Hinaazu evolves into Doduo, the third head gets eaten by the other two. And when Doduo evolves into Dodrio, the third head comes back as the angry head.

One similar theory I've read is that when Hinaazu becomes Doduo, the soon-to-be sad head is hiding in the body's feathers. Then once it becomes a Dodrio it's forced to pop out because it's gotten too big.

Hanae said:

One similar theory I've read is that when Hinaazu becomes Doduo, the soon-to-be sad head is hiding in the body's feathers. Then once it becomes a Dodrio it's forced to pop out because it's gotten too big.

Or, perhaps one of the heads' points downward and its beak becomes Doduo's legs? No one ever pays too much attention to their legs; it could be a modified beak!

couldn't it be argued that the third "head" on hinaazu isn't a real head but a decoy?