http://www.eaudrey.com/myth/index.html

From the page:

“Description
A fish that appears like a bishop. This idea may be based on the belief that everything on land has a corresponding creature in the sea, or may have been a fitting description for an unfamiliar creature that was caught. Also called a Bishop Fish.

Might be… a giant squid. Squids have heads shaped like a bishop, and it is not uncommon for them to be the size of a man. see: http://unmuseum.mus.pa.us/squid.htm

Described By: One such creature was seen of the coast of Poland in 1531.

In a reported Medieval legend, a Sea Bishop was captured alive in a fishing net in the 13th century. When it was brought to the king, the Sea Bishop begged to be set free. It was brought back to the sea, and before diving into the water it made the sign of the cross.”