"Ein Delfin ist ein Säugetier."

Translation: A dolphin is a mammal.

[zengator]

Auf Englisch, ein "Dolphin" kann auch ein "Dolphinfish" oder "mahi-mahi" oder "dorado". Auf Deutsch, das ist eine "Goldmakrele".

[G.P.Niers]

That sense of ‘dolphin’ is really rare though. In practice, when people are talking about dolphins they almost always mean the mammals.

[zengator]

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.

[TomRaptile]

Why is Delphin not accepted. Isn't it an alternative spelling?

[zengator]

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:

• Duden
• Wiktionary

[TomRaptile]

Yes it was supposed to be the only mistake.