"Why are you looking for that horse?"
Translation:Proč hledáte toho koně?
Hmmm.... this is odd. Will need to be researched.
UPDATE ----- This is an intermittent grading bug affecting multiple courses. Duolingo staff are working to identify the cause and find a solution. Contributor teams cannot do anything to resolve the problem directly, apart from confirming that a rejected translation should be accepted.
You've got to understand the function in the sentence. 'kůň' is nominative case, and is used for the subject of a sentence: 'Ten kůň je velký'.
In the sentence for this exercise, 'vy' (in the formal or plural form in the preferred translation) is the subject of the sentence, not stated but implied by the conjugation of the verb, 'hledáte'; that horse is what you are looking for and is in the accusative case (hledat takes objects in the accusative). Hence, toho koně.