Recently, I learnt that the literal meaning of the main written Japanese form for dolphin, "海豚", is "sea" and "pig". The Japanese person mentioning that noted that he didn't know for sure why that was the case. At first I thought it was because dolphin meat was similar to pork, but then I looked further into it.
海豚 is also used in Chinese. Meanwhile, the English word "porpoise", which comes from Medieval Latin via French, is based on a compound of pig and fish. In addition, the Welsh for "porpoise" is based on sea pig and sea sow.
Online Etymology Dictionary mentions that Latin had "porculus marinus", with the theory that its snout looked like that of a pig.
The Wikipedia article Marine mammals as food mentions that in Peru, dolphin meat is referred to as "sea pork".
I came across the fact that the German for guinea pig is "sea pig", but I'm not looking into that.
Why are some sea mammals called "sea pigs", especially in Chinese/Japanese? Failing that, has there been any cases of sea mammals being called "sea pigs", where it is known that the speakers of that language don't eat sea mammal meat?