Taroumaru

Taroumaru is a little dog that Yuki found while exploring. After some begging, he was allowed to stay in the club.

In the manga:

Taroumaru only appeared once in a flashback before the story began. Upon examining him, Megu-nee found a bite mark on his neck and concluded that animals can become infected as well. Megu-nee kept him separate from the girls to observe his condition, and took him outside when he turned.

In the anime:

In episode 9, he becomes infected after breaking free of his leash at night and wandering off to one of the barricaded zones of the school. He is supposedly bitten by Megu-nee.

In episode 12, Taroumaru saves Yuki and she brings him back to the club room. They used the zombie vaccine on him, and he returned to his old self. But, unfortunately he didn't last very long and was buried next to Megu-nee's grave in the school garden on the roof.