Arwoo doesn't speak. He is a pet. Karli insisted upon naming him, which presented a problem because she can't see, and also because she's a small child. She named him after the sound he makes. "Arwooo". He is small He is cute His full name is a nightmare. Arnold- the gang trying to find a way to name him something socially acceptable, derived from "Arwoo" Little Croissant- Maybe that's what Klik said in morse code. Maybe he wanted to eat Arwoo. Whisker- Because he's part cat, and...not much else Buddy- Average dog name. The Third- Paul. Paul didn't want to contribute.
