Crazy Old Friedrich is the "kind old gunner" (12.6) in charge of Joey and the rest of the horses taken from Emilie by the Germans. Joey spends more time with him than he does with Nicholls, Stewart, Warren, or any of the British in battle. That means Joey really gets to know him—and wonders who the crazy one is in war.
Joey describes Friedrich as "the only one who understood himself" (13.6). All the other soldiers tend to stay away from him because he talks to himself, but he likes it that way. He's sensitive, a pacifist, and hates all the killing. The irony here is that he was a butcher in his civilian life. We guess if they were fighting a war against armed cattle, Friedrich wouldn't be too conflicted about it.
Friedrich's sensitive side comes out again while he bonds with Topthorn as a kindred misplaced soul. When Topthorn is killed, Friedrich shouts one of the novel's most heart-wrenching (and melodramatic) lines: "Why does this war have to destroy anything and everything that's fine and beautiful?" (14.5)
What can we learn from this? No one on either side—neither the British nor the Germans—want to be at war. Sometimes in English-language art and literature, the Entente (or Allied) powers are portrayed as the "good" guys, while the Central Powers, including the Germans, are depicted as the "bad" guys. But through the eyes of Joey the war horse, we see that a lot people were just stuck in a war out of their control.