Shibashiba

Shibashiba is a large hairy black monster bird who everyone on Angel Island is afraid of (though Kyoro-chan himself sometimes isn't). It can shapeshift and reproduce itself, and it can turn someone else into a Shibashiba (as once happened to Mikken).

There seems to be some religious connection to the Shibashiba, as the townspeople once sacrificed Kyoro-chan to it (actually, he volunteered, but ended up making friends with the Shibashiba anyway). The only thing that it can say is "shibashiba", hence its name. If you are turned into a Shibashiba, all your speech will come out that way. Mikken's grandma seemed to be able to recognize Mikken even when he was turned into a Shibashiba. Somehow this recognition turned him back. So it's possible that if someone realizes who you really are, you stop being a Shibashiba.

The name Shibashiba has two meanings: (1) often/frequently/again and again, and (2) blinking repeatedly. The second is more relevant, since the names of Kyoro-chan and some of his friends refer to eyes in some manner (Kyorokyoro = looking around restlessly, Pachikuri = blinking with surprise, Miken = area between eyebrows, Masukara = mascara). Though it's possible that the double meaning is intentional, since episode 43 is called "Matamata, Shibashiba" meaning "Once again, Shibashiba" or literally "Once again, again and again". The Shibashiba does show up multiple times in the series too, which also makes that meaning more relevant.