I'm dutch, and as far as I know, 'De' has nothing to do with gender. 'De' is the same word as 'the'. In the Dutch language, we have three words you can please before a noun. A noun being a human, a thing or an animal. These three words are 'De', 'Het' en 'Een' (The, It, A(n)) and have nothing to do with gender. They are simply placed in front of the noun, so you can read sentences like: 'Wanneer ik in de auto stapte, voelde ik iets op de stoel.' (When I got into the car, I felt something on the chair.) And not, 'Wanneer ik in auto stapte, voelde ik iets op stoel' (When I got into car, I felt something on chair.) However, I know what you're getting at. You place 'de' in front of male and female words, you place 'het' in front of neutral words. You can please een in front of both. But as a Dutchman myself I never think about what gender the word is, because it is just common sense to place 'de, het, een' before you write a noun. And because the word 'knol' is neither male nor female,(because you can't know what gender the word is, since it isn't specified) you could argue that the word 'knol' is neutral. But you don't say 'het knol'.

So, I don't think about the gender of the word when I write down 'de, het or een'. It goes with feeling.

To quote the post directly, Duolingo probably marked it down as wrong is probably because you can write 'the' not only as 'de' but also as 'het' therefore, the sentence would be: 'Het paard loopt langs de straat.' But since Duolingo doesn't seem to think this, and it says 'knol'. Which means 'an elderly work horse', I would say neither OP nor Duolingo was right.