The Soul Reaper Captains of Bleach's Gotei 13 are some of the most powerful fighters in that series' universe, and there's plenty of details that set these characters apart from one another. One character, Sajin Komamura, stands out for a very different reason, however: his inexplicable wolf head.
In Komamura's initial appearances, the Captain of the Seventh Squad doesn't speak at all, and wears what appears to be a wooden helmet over his head, completely obscuring his visage. While this air of mystery does make him seem suspicious in the wake of the revelation that a traitor is in their midst, the truth is simply that Komamura is self-conscious about his looks. So far as the audience is shown, he's the only person in the Soul Society to look like this, so it's understandable. Komamura turns out to be both a powerful and loyal Soul Reaper, becoming one of the first to stand against Aizen after his betrayal is revealed.
Komamura continues to appear alongside the other Soul Reaper Captains, but the nature of his appearance isn't explained until the Thousand Year Blood War arc, which begins just after where the anime adaptation of Bleach ended. In chapter 538, Komamura pays a visit to a suspicious cave, where he meets a massive wolf-like being whom he addresses as Grand Elder. He demands to know the secrets of their clan, and is met with derision because of his desire to conceal his appearance and associate with ordinary human-souls. What goes down isn't explained until chapter 556, in flashback: the Elder explains that they're werewolves, and they were made this way because of the sins their clan committed during life in the human world, and were sent to the "Beast Realm." They couldn't help but desire to be alongside fellow humans again, and the end result was to wind up somewhere in between beast and man.
As it turns out, it's possible to temporarily regain their human appearances, and with it access an immense amount of power--all one has to do is rip their own heart out. Komamura obliges the Elder's request out of his sense of obligation to Head Captain Yamamoto, who allowed him to join the Soul Reapers in spite of his cursed nature. The manga then cuts back to the present, where his armor is removed and he does indeed look like a human. By separating his heart from the body, the body becomes both human in appearance and completely immortal, providing him a massive advantage in the ongoing battle with the Quincy. It even changes the appearance of his zanpakuto's Bankai ability, causing it to look more human as well. While this gives him the edge he needs to win, the consequence is that his body transforms into that of a regular wolf, and Komamura winds up out of commission for the rest of the war.
The lore and cosmology of Bleach's world is rich and complex, and Komamura's werewolf clan is an example of one corner that largely remains unexplored. The aforementioned Beast realm hasn't come up alongside the other spiritual worlds like Hueco Mundo or the Soul Society, so details on it remain quite vague. Is it a world where the souls of animals go? Since the clan was able to move from the Beast Realm to the Soul Society, it must be possible to travel between the two. And, since reincarnation does exist in the world of Bleach, is it possible for souls that were human to reincarnate as animals, and then wind up in this Beast Realm? With a possible revival of Bleach on the horizon, fans may yet find out.