Here's my theory. So Winds and Waves is based on Indonesia right? Well that's where chickens come from. Before chickens were domesticated they were jungle fowl in Indonesia. Indonesia is specially known for creating an all-black breed of chicken called the Ayam Cemani.
I think Browt will evolve to be a dark grass type based on the Ayam Cemani. "But wait," you say, "He's not all black." First of all, his final evolution has to be dual type. He has to have colors other than black. Note his black toenails on black legs. That detail stood out to me. Making them white seems like the more obvious design choice. The black on black feels intentional. The slight green iridescence on the ayam cemani's tail feathers is also interesting. They could incorporate that to bring in the grass type.
"But they've already made a chicken Pokemon–Blaziken!" I think they did him dirty. He's too human-like. The time has come for a more chicken-y chicken.
But also, I'm getting chickens in three weeks, so I might just be seeing them everywhere…
by throwawayvacayday