The words for male and female in Russian are самец and самка. Самка is an uncomplicated word with genitive plural самок. Самец is an end stressed word:

In English we might say "A rooster is a male chicken" or "A mare is an adult female horse," but we have to rephrase it slightly for the Russian version. In Russian one says «Петух — самец курицы» “A rooster is the male of the chicken.” In other words, the word that follows самец or самка needs to be in the genitive case. Likewise «Кобыла — взрослая самка лошади» “A mare is an adult female of the horse.”