"That crab is eating a mouse."
Translation:Tamten krab je mysz.
Why not 'Tamten krab je mysza'? I'm aware that mysz is fem., but shouldn't it be changed according to masculine-animate noun's rules? I would say 'Tamten krab je kota' for example, so with mouse I expect the same.
No, because as You noted, mysz is feminine noun, ending in sz, which makes the declension pattern somehow weird – Accusative here is the same as Nominative.
BTW, due to small size of the animal, most people would use here a diminutive "myszka", which would make it "Tamten krab je myszkę" – pretty much the only time I use "mysz" is either in plural(Tu są myszy) or for the computer input device(a mouse ;-) ).
Interesting. And yet, masculine nouns that end in -a (e.g. mężczyzna) 'behave' like feminine nouns throughout, right?