"Het dier is een koe."
Translation:The animal is a cow.
Well, if it was: "Dat" dier is een koe", it would have made more sense. But "the animal is a cow" is kinda strange, since all cows are animals, yet not the other way around.
I see what you mean, but by using a definite article ("het") you also indicate that you're talking about a particular animal, which happens to be a cow in this case. "Een dier is een koe" would be (semantically) strange.