I guessed right! The Titan was Eren!
How did he turn into one though? 0.o
I Can Hear His Heartbeat: The Struggle for Trost, Part 4
Episode Summary (from Wikipedia)
With the supply depot area swarming with Titans, Armin has the idea of using the Rogue Titan to defeat them. Mikasa and Conny agree with his plan and defeat the Titans attacking it, before rushing there. Meanwhile, Jean and the others reach the supply depot despite suffering more casualties on the way, with Armin, Mikasa and Conny arriving soon after. Armin's plan works, but while the Rogue Titan fights the enemies outside, seven smaller Titans lurk around the fuel storage area. With their gears out of fuel, Armin comes with another plan. The survivors draw the Titans' attention to an elevator and shoot them in the eyes to blind them, while seven of them will jump from the ceiling to finish them off. Sasha and Conny fail to kill their targets but are saved by Mikasa and Annie. Having successfully replenished their fuel, the soldiers prepare to retreat, but they see the Rogue Titan defeating all of the other Titans before collapsing, and much to their shock, watch Eren emerging from its body. Mikasa reaches Eren and cries upon confirming that he is alive, while Armin notices that somehow his severed leg and arm have regrown as well.
My thoughts as I watched this episode:
>>> Okay Armin seriouslly needs to get himself together...
>>> Aaaaaaand he comes up with a genius plan right after I criticize him...smh.
>>> The might-be-Eren Titan has abs lolololol
>>> Jean finally being a good leader...YESHHH!
>>> OMG these Titans need to stop!
>>> Yes, this Titan is awesomeeee!
>>> Come on people, it is Eren (I think)!
>>> Was Armin always this smart? Dang.
>>> Armin be confident!!!
>>> OMG Sasha is apologizing to the Titan...HAHAHAHAHA
>>> Jean is so adorable lol
>>> Save the might-be-Eren Titan! Don't let him die!
>>> OMG IT'S EREN IT'S EREN IT'S EREN IT'S EREN!!!