The accusative of the indefinite article
The same phenomenon came be observed when the indefinite article is used:
1) Ein Hund mag eine Katze.
("A dog likes a cat.")
2) Eine Katze mag einen Hund.
("A cat likes a dog.")
The accusative of "kein"
Logically enough, the negative "kein" declines in exactly the same way as "ein" in the accusative:
1) Ein Hund mag keine Katze.
("A dog doesn't like a cat.")
2) Eine Katze mag keinen Hund.
("A cat doesn't like a dog.")