As much as Teen Titans was known for its mature storylines, it was also known for its comedy, and the episode "Every Dog Has Its Day" was definitely one of them, and it introduced Soto.
Soto was an alien who had a dog...that was green...see what's coming? One day, Beast Boy wanted to get attention, so he became a dog. Little did he know that Soto's dog was "missing" in the city too, and when Soto found BB, he thought he was his dog.
Now, Soto wasn't technically a bad guy, but he did do some mean things to Beast Boy because he thought he was misbehaving. Thankfully, everything got resolved and everyone got back to where they should be.