There are countless ghouls in Tokyo Ghoul, but the most powerful of these dangerous entities receive unique code names. Ken Kaneki is known as "Eyepatch," Touka Kirishima is "Rabbit," and Renji Yomo is "Raven." However, several characters throughout the series have the code name "Owl," which sounds incredibly confusing at first. That said, the term apparently originated with their unique kakuja, which supposedly took on an Owl-like appearance.
Source：https://www.cbr.com/tokyo-ghoul-the-owls-explained/
