I was confused when I looked up 雌（メス）on jisho and found it was on WK, then 雄（オス） after and found it wasn’t on wanikani. I’m on the level where you learn the kanji and you learn two vocabs with it, 英雄 and 雄犬. I’m really confused as to why the kun’yomi appears in one of the words but they didn’t put it as a word itself although it’s commonly used and the female equivalent appears later.
I don’t think there’s any deep meaning. They teach the readings めす and おす, and they teach words that show both words in use, even if they aren’t the same types of examples for both words.