Spherical cow
The words "spherical cow" came from a joke where a dairy farmer talks to a theoretical physicist about how to get more prices for his milk. The physicist later talks about an answer to the problem at a seminar. The physicist says "First, we assume a spherical cow..." Sometimes, a "spherical horse" is used instead of a cow. For example, there is a joke about a physicist who says that he can find out who will win a race if it had a "spherical horse moving through a vacuum".