cherydactyl

SF Two Cow jokes from http://practical-tech.com/entertainment/two-cow-jokes-the-science-fiction-ones/#more-1511, where there are lots more of them, but here's a sample. Many of them had me laughing so hard I was tearing up. Noticed via leguin, thanks!

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Ursula K. Le Guin’s Earthsea Trilogy – You have two cows. You can only get milk out of them if you know their real names.

Ursula K. Le Guin’s The Left Hand of Darkness - You have two cows. Sometimes you have two bulls instead.

Lord of the Rings - Two cows to rule them all.

Sword of Shannara - Two cows to rule them all.

Mad Max - Two cows enter. One cow leaves.

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I keep thinking I need to write one for PTerry's Discworld. Twoflower, Two Cows....??

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Edited to add some contributions by dspitzle:

Jasper Fforde: You have two cows. You trade one for some magic beans, and the other jumps over the moon

Douglas Adams: You have two cows, and you left yourself a message you can only read by superimposing their spots

Daniel Pinkwater: You have two cows, one of which is actually Mozart.