The joke is that no matter how simple and basic the lesson there will always be the guy that has questions.
“Ok class we’re going to start simple. This is a cow.”
hand immediately raises
“Oh wow. Ok. First slide and there’s already a question. Yes, you in the back what’s you’re question?”
Whether Larson knew this and was drawing an anatomically accurate horned cow or was under the common misconception that cows lack horns and was trying to make a double layered joke about "someone has a stupid question about everything, even a simple picture of a cow" in conjunction with "but actually if you look closely enough there's something to question about this particular cow picture" I dunno.