"A fish and a dog are swimming."
Translation:Ein Fisch und ein Hund schwimmen.
‘Einen’ is the accusative form of the masculine article, used for direct objects. The nominative form, used for subjects, is the same for neuter and masculine (i.e. ‘ein’). Here both ‘fish’ and ‘dog’ are subjects, they both perform the action of swimming, neither of them is ‘being swum’.