The Wall Titans were Abnormal Titans similar in appearance to the Colossus Titan, one of the Nine Titans. They possessed the ability of crystallization and existed in vast numbers composing the three Walls, before Eren Yeager freed them in the year 854 via unhardening the Walls.
In the year 743, King Karl Fritz used the power of the Founding Titan to create and command "tens of millions" of these Titans to create the three Walls (Maria, Rose, and Sina) and protect the last Eldian territory on Paradis Island. The lack of sunlight inside the Walls has supposedly rendered them immobile ever since, although King Fritz "threatened" to release them and use them to flatten the Earth should Paradis Island come under attack.
Their existence was discovered when the Female Titan tried to climb Wall Sheena to escape from Eren and the Survey Corps.
Appearance: These Titans were similar in appearance to the Colossus Titan in that both their surfaces were covered primarily in muscle tissue and lack skin (although the Colossus Titan typically had more bony tissue exposed around its face). Like the Colossus Titan, they all shared the long arms and elephantine feet necessary to support their structures. Each Titan varied slightly, most noticeably around the face; some possessed very defleshed faces, with little more than bare skulls, while others exhibited complete musculature.
Abilities: Like other Titans, the Wall Titans generated great amounts of heat, enough to emit large plumes of steam not unlike the Colossus Titan. They could use this ability both offensively and defensively, creating smokescreens and flash-boiling seawater to incinerate entire battleships. All Wall Titans possessed the ability to harden, although it was probable that they only did so because they were commanded to by the Founding Titan.
History: In the year 743, after King Karl Fritz abandoned the conflicts of Eldia causing the Great Titan War, he fled overseas with the main branch of the Fritz royal family and remnants of the Eldian populace to Paradis Island, Eldia's last remaining undisputed territory. He used the power of the Founding Titan to turn his subjects into tens of millions of Wall Titans to form the three concentric Walls to protect the rest of the Eldians. " Over 80 years later, Dina Fritz explained this story to the Eldian Restorationists.
Story: During the battle between Eren Yeager and Annie Leonhart in the Stohess District, Annie attempts to escape by climbing the Wall but is stopped by Mikasa Ackerman. However, she damages part of the Wall, and a huge chunk is torn off, revealing the face of a mysterious Titan. The massive hole in the Wall is immediately noticed by Mikasa.
Source: Wall Titan, available under CC BY-SA 3.0.