"They do not see this horse."
Translation:Oni nie widzą tego konia.
In the idea that "koń" is feminine. I guess there are words ending with -ń that are feminine (although nothing comes to my mind now), but mostly they are masculine. And "koń" is masculine. If it was feminine, I'd guess it'd be "tej koni".
Two mistakes. One, 'widzemy' is not any existent form, you must've meant "widzimy". But that is 1st person plural: My widzimy = We see.
"One" are 3rd person plural. "One widzą" = They see.