The beautiful aspect of naming conventions would be that they are always inclined towards change due, in no small part, to societal progress. Thiago is one of those steady names that derived from the diminutive for “Santiago.” Now granted, the name may be in full abundance in certain countries in Latin America, looking at you Brazil. But here in the USA, thankfully, it still remains uncommon and therefore unique. Cheers!
This name is literally everywhere in Brazil. In fact, I have a nephew named Thiago, and there's other Thiagos in my family too. But at least over here it's been used for a long time. I kinda wish this name stayed in Brazil. Since Lionel Messi used it on his son it's exploded in popularity abroad. It's definitely too trendy at this point.
Thiago Alves d'Araujo is a Brazilian mixed martial artist currently competing as a welterweight in the Ultimate Fighting Championship. As of March 7, 2016, he is #15 in official UFC welterweight rankings.
Thiago Furukawa Lucas, who goes by his pen name Yuu Kamiya, is a Japanese-Brazilian novelist and illustrator. He is known for his manga "No Game No Life".
This name, the Portuguese form, comes directly from Tiago that comes from Santiago that comes from Santo Iago, and the Iago comes from Iagus, and etc. Diego is not related to Tiago and the other variations. Diego, as Tiago, comes from Iago.