Latino is originated from the people of Italy (Romans used to speak latin, remember?). So any language that originated from Latin (Italian, French, Spanish, Portuguese) the people that spoke these are the real latins.
That means that Latinos are europeans that speak Italian, French, portuguese or Spanish. So when the continent of "America" was conquered by spaniards, they all called their indigeneous slaves "latinos"
Mexican1: I´m a proud latino!
Mexican2: Yeah! I love being a slave too!