"The cat and the dog are running onto the street."
Translation:Die Katze und der Hund laufen auf die Straße.
"Auf" is a two-way preposition, which means that the subsequent article can be either accusative ("die") or dative ("der") depending on context.
The notes on this exercise (which are available on Web and iOS but not Android grumble) state that "If there's movement from one place to another, use the accusative case", and "If there's no movement, or if there's movement within a certain place, use the dative case."
In this case, there is "movement from one place to another" implied, which results in the accusative "die Strasse". Does that answer your question?
(Answering this question helped me understand two-way prepositions better, so thank you!)
In one case one has the horses "on the street", being on the street to begin with and still on the street (same place) - dative for the two-way preposition. In the other one has the cat and the dog running "onto the street", that is, starting OFF the street and proceeding ONTO the street (from one place to another) - accusative for two-way preposition.