"We have a dog and a cat."

Translation: Mamy psa i kota.

[AbdullahAl170582]

Why do we use psa not pies?

[Latcarf]

Because "dog" and "cat" are direct objects of the verb so they have to be in the accusative case. And the accusative of masculine animate nouns is formed by adding -a at the end (and removing -ie- in the case of "pies")

[BenConway6]

It seems that the accusative form of most masculine animate nouns is the same as the genitive, including in the adjective. Mam małego psa, for example.

[Alexine761464]

I was sure that the accusative was psem and kotem... God i'm lost now

[Jellei]

No, those are Instrumental forms, as in "This animal is a cat/dog" (To zwierzę jest kotem/psem).