Something I'd never learned before I read the article on "euphemism", titled Euphemism, by Neil Postman is that "horse mackerel" and "tuna fish" should refer to the same thing! And according to the author, "horse mackerel" is a name less favored than "tuna fish" by ordinary customers when they are buying this particular race of fish. However, I Googled both "horse mackerel" and "tuna fish" for their pictures when I found in these pictures that there do exist some differences between the fishes the two names refer to respectively. Now, I'm still wondering if these two names refer to the same fish in the minds of the people who eat it often as a usual dish. Thank you.