User blog comment:Sim0n2170/Attack on Titan Season 2 announced for SPRING 2017/@comment-29546530-20160809034023/@comment-5774380-20160809035007

"Maybe the titan that was in the wall is named after the wall it was in, so it could be Rose or the one that surrounds the interior ( I forgot it's name ). The titan might also have been kept a secret by the people who made the wall, or that cult the worships the walls as gods so it won't scare everyone else."

I can't reply to this without spoiling you, so I'll just say that you should read the manga. :P Or wait for Season 2 which will reveal the answers to these questions.