The mysterious "inzibyi"

We were talking to a young Rwandan friend last night. He has often ben out fishing for sambaza, small sardine-like fish which are very popular in the market here. They are attracted by lights and then caught in nets. The presence of dozens of fishing boats in a small area of lake gives it quite a magical quality most nights.
In the course of the conversation, he mentioned that they also sometimes caught an animal in their nets - the "inzibyi", English / French / Swahili names unknown. This was described as :

about 20-30cm long
black, with a large mouth
like a small pig - an animal with legs, not a fish
capable of biting people
not eaten by humans, but given to dogs to eat

I am intrigued and have no idea what this creature is, although its description and name was confirmed by several others in the company. Suggestions welcome, I am going to try to find out.