The most famous sea monk supposedly washed ashore in Denmark in 1546 and became famous all over Europe. The bottom drawing is based on depictions of that particular sea monk. The top drawing is based on monks' own depictions of the creatures.
Sea monks tended to either be peaceful or, oddly, act like mermaids rather than mermen. You see, the mean spirited kind lured people into the sea and ate them, just like mermaids. (Mermen rarely did anything besides getting caught and predicting the future)