Hodor can say only one word. This has been true since season 1. Hodor for this. And Hodor for that. But as the new season has shown, Hodor wasn't always like this. In fact...

Hodor could talk! Bran Stark saw a vision of the past, and found out Hodor's real name - Wylis.

And... Wylis could talk!

According to redditors u/TazoGreenTea and u/gbinasia, Hodor might have been a warg like Bran.

“I’m thinking that Wylis went far beyond the limits of warging which may have mentally damaged him and turned him into Hodor,” theorizes TazoGreenTea. “I remember Jojen Reed warning Bran not to warg for too long or else he’ll be trapped and forget he’s human.”

The theory is that Hodor might have chosen to warg into his favorite animal - a horse. And he might have did that to spend more time with Lyanna, since Lyanna was very kind to him.

Explaining why Hodor is comfortable carrying Bran on his back using a saddle.

Also, Bran can warg into him. Is Hodor an animal? Is he Lyanna's horse?

Was he warg?

Hodor.