Pakka (Partial Kappa Transformations)

Pakka is a manga by Daisuke Imai about a high school swimmer who on several occasions partially transforms into a kappa (an aquatic monster from Japanese folklore that has the form of an anthropoid reptile).

His first experience as a partial kappa happens in chapter one, while he's in the school pool:

In chapter two he starts transforming in class and rushes to the bathroom:

In chapter 31 he transforms in front of friends: