"The cow is not wearing a hat!"
Translation:Die Kuh trägt keinen Hut!
The form of "kein" matches the gender of the noun "hat" which is masculine and it's the accusative case which makes it "keinen".
If the use of kein, keine, or keinen, is to negate a noun, and nicht to negate a verb, in the sentence, "The cow is not wearing a hat", the part that is negated is the verb. Is the cow wearing? No, the cow is not wearing, hence nicht, yes? "Die Kuh trägt keinen Hut" implies the cow is not wearing a hat but rather something else.
Once again I would like to report that the pronunciation of the individual word "Hut," when you move the block containing that word, is wrong! He is saying it to rhyme with English nut or gut. Needs a long oo, as in boot. I DID flag this but the specificity of why I flagged what isn't carried by a simple menu of broad choices.