Just another example of random internet searches being useless...

In high school, I took a Spanish class. In college, I took a Spanish class. I took an online Spanish class and I've listened to books on tape accompanied by actual books. I have plenty of friends who speak Spanish. I don't speak Spanish. It just has never stuck.

That random gibberish out of the way, I am trying to determine why one says "el pollo loco" (the crazy chicken) but then says "el loco payaso" (the crazy clown). If anyone can tell me why this is not an absolutely arbitrary grammar rule, I'd be happy to hear it. Or more preferably, a detailed explanation of why Spanish grammar works this way.