Some animals have masculine words ending in -o and similar feminine words ending in -a.

el gato the cat	la gata the cat
el cerdo the pig	la cerda the pig

Other animals have completely different words for the male and the female gender.

el caballo the stallion, the horse	la yegua the mare
el toro the bull	la vaca the cow

And finally, there are animals which have only one grammatical gender, whether they are male of female.

el pez the fish	el pez the fish
la mosca the fly	la mosca the fly

We can use the words macho and hembra to distinguish between the male and female animals that have only one grammatical gender.

el pez macho
the male fish

el pez hembra
the female fish