Does not have to be actually, but you are correct we do require a demonstrative pronoun in most sentences for the.
If you have one goat at home and you say "Koza žere žrádlo.", it is clear which goat it is, it is "the goat".
But this should relly belong to the opposite direction. This one is Czech -> English.
So, a goat is certainly possible, but the goat is possible as well.