Why is the plural form of "koń", "konie"?
To answer that, we need to learn a few rules:
Polish has "hard" consonants and "soft" consonants. Depending on if the word ends in a hard or soft consonant, it can change what ending the word takes in the plural. (and a lot of other things, too.) You can find many complete lists of hard and soft consonants online.
"Koń" is an inanimate masculine non-personal noun, which means that the NOMINATIVE and ACCUSATIVE plural is formed by adding "y" to the end of a word if the word ends in any hard consonant except "g" and "k" (ex. Kot -> Koty), adding "i" if the word does end in a "g" or "k" (ex. Ptak -> Ptaki), or by adding "e" if the word ends in a soft consonant. (ex. hotel -> hotele)
In this case, "koń" ends in a soft consonant, "ń", so the plural ends in "e".
So why is it "Konie" instead of "Końe"?
That's because Polish also has some spelling rules: you cannot write "ń" before a vowel. You must replace "ń" with "ni" before a vowel. (Similar thing applies to other kreska consonants, such as ś -> si, ć -> ci, ź -> zi...)
Thus, the plural of "koń" is "konie".
If you don't understand, have questions, or think I've made a mistake, please reply.
Koń is a horse, singular. Konie are horses, plural. They are also pronounced differently – koń is only one syllable, while konie has two syllables with clear /ɲe/ at the end.