"The crab is an animal."
Translation:Krab jest zwierzęciem.
Do not discard your sentence yet. Incidentally, it is correct, but not here. It is the little "to", which gives the sentence a totally different meaning: "To krab" means "It is the crab which...", so "to" doesn't define the crab, but indicates the crab and distinguishes it from the crowd:
To krab jest zwierzęciem - It is the crab, which is an animal (others are not)
Whatever is after "to" - doesn't change. After "jest" - nouns change. A correct sentence with it is "Krab jest zwierzęciem".
This is called the instrumental case. Masculine and neuter words end with -em, feminine with -ą (including all a-ending masculine nouns such as mężczyzna).
My personal view is that "Krab to zwierzę" is more like "A crab = an animal" and "Krab jest zwierzęciem" is more... descriptive. In most cases the Instrumental form sounds superior to me, although a moment ago I was commenting on "This animal is a tiger" and I'd use "To zwierzę to tygrys" there... because "This animal = a tiger" makes more sense there.