"The horse" is an accepted answer (starred even), it definitely should have worked.
On one hand, it's easy to say that "ni" and "ń" is the same sound. But they are a bit different.
As in 'ni' you have 'i', it is pronounced in a way. It's a bit like "ńi".
"ni" is written before vowels, "ń" before consonants and at the end of the word - at least the word in Nominative.
As actually "koń" is Nominative singular, but "koni" is Genitive plural. Too early to discuss Genitive, I guess ;)
Well, it's for sure the closest that you can probably get to the Polish ń from the knowledge of other languages. But I'd say that it's shorter than the Spanish one.
horses do not like milk, it gives them diarrhea, unless 'foul' and 'horse milk' was meant?