you actually got it reversed, but I see your point.
Mulato (one L, one T) refers to people. That word also USED to refer to mules as well, but the actual word for mule in spanish is just Mula. The spanish language doesn't have double T's, and the double L would be pronounced like a J or a Y.
And the word Mulato DID in fact, come from the whole mule-concept ting. The analogy of mulatos and being mules comes from how people viewed horses and donkeys at the time. Horses were elegant, graceful and beautiful creatures, whereas donkeys are hardheaded, stupid animals only suitable for labour. The Spanish projected these views onto their comceptions of race (Horse vs. Donkey= White vs. Black) and thus the term Mulato was born for people who were of mixed Black & White racial heritage.
Its kinda f*cked up, innit? But I didn't know it causes debate or anythin like that lol...I just thought people couldn't spell good.