Auf Englisch, ein "Dolphin" kann auch ein "Dolphinfish" oder "mahi-mahi" oder "dorado". Auf Deutsch, das ist eine "Goldmakrele".
That sense of ‘dolphin’ is really rare though. In practice, when people are talking about dolphins they almost always mean the mammals.
Not all that rare in Florida. In fact, I think some folks find it entertaining to see the horrified/confused looks on the faces of people who look on a menu and find they can have dolphin broiled, fried, grilled, and even blackened.
Why is Delphin not accepted. Isn't it an alternative spelling?
Assuming every other word in your answer was correct (it helps to copy/paste your full answer) then yes, Ein Delphin . . . should have been accepted. Reference:
Yes it was supposed to be the only mistake.