"I know a cat."
Not only that, but this feline will make a beeline straight for this offer. One witty kitty.
No, it's Accusative, as needed by "znać". Although Genitive would look identical: Nie znam kota.
But it actually isn't. In Accusative of masculine nouns, it matters whether the noun is animate or inanimate. This is the only situation where it matters.
If it's animate, then Accusative is identical to Genitive. "Widzę kota" = "I see a cat" (Accusative), "Potrzebuję kota" = "I need a cat" (Genitive).
If it's inanimate, then Accusative is identical to Nominative. "Widzę stół" = "I see a table" (Accusative), "To jest stół" = "This is a table" (Nominative).