"The horses are not big."
Translation:Die Pferde sind nicht groß.
Kein is used to negate a noun whereas nicht is used to negate a verb. For example:
"I can't see"
"Ich sehe nicht"
Nicht negates the verb see/sehe. Whereas
"I don't see any cheese"
"Ich sehe keinen käse"
Keinen negates the noun, it is the cheese that you can't see.
Kein is just the same word as the definite article ein, just with an added 'k' and you can only use the definite articles with nouns. You can have THE cheese but not THE see, i hope this makes sense and helps you out.
Thank you LukeCox13. Thats the first explanation I've seen that has made it really clear for me. Much appreciated.