Well the cow level had ties to the culture of Diablo. It was a throwback to the rumors in Diablo 1 that there was a cow level, people said things like if you click the cows in a certain pattern or something you'd get a portal to the cow level. That's where the entire idea came from for Diablo 2. I have no idea where the idea for the pony level came from, other than the stupid bronies fad lately.