Abnormal

An "Abnormal" (奇行種) was an uncommon type of Pure Titan which could possess unusual intelligence or perform unpredictable actions.

Description
Unlike the typical Titan that attacked and devoured humans on sight, abnormal Titans might choose to ignore single or isolated humans, and exhibited unpredictable behavior that could lead to greater damage as they disrupted important strategic locations. Some of these uncommon behaviors were speed-walking, running, performing great jumps, and even speaking human languages.

Ilse's Notebook
During an expedition outside the Walls, Hange Zoë among other members of Levi's squad find a lost diary on the ground that belonged to Ilse Langnar before her death. The diary explains that after Ilse's entire flank was massacred, she fled to a nearby forest, heading north. After profuse running, she inadvertently runs into a Titan and writes in the diary, believing it to be her end. She remarks that instead of immediately eating her, it hesitates. It then addresses her, saying "a subject of Ymir" and "Lady Ymir," greeting her with deep respect, and bows. She is thoroughly confused that the Titan can form speech, and writes in her diary that she has attempted communication. She first questions it about what the Titans are, and building up blind sadness, fear and rage, she angrily yells why Titans eat humans and what their objective is. The Titan appears more and more frustrated as it moans and tears at its cheeks, ripping off flesh and spraying blood. Ilse attempts to flee, but is caught by the Titan, who crushes her skull in its teeth while seemingly crying.

Battle of Trost District arc
As Eren Yeager's group was heading towards the middle guard in the Trost District, an abnormal Titan jumped out of nowhere and grabbed Thomas Wagner in its mouth. The rest of the group looked on, horrified, as the Titan swallowed him whole. The same Titan would later appear, where it would be killed by Eren in his Titan form.

Another abnormal Titan appeared and charged straight towards the rear exit of Trost, where a group of citizens were gathered and trying to escape. It ran so fast that the elite guards stationed in that area could not keep up with it. Mikasa Ackerman appeared and sliced the nape of its neck to kill it, dulling her blades in the process.

The Female Titan arc
The Survey Corps went on its expedition outside Wall Rose and on the way, an abnormal suddenly appeared towards the right flank. It completely ignored the soldiers at the outer guard and charged straight towards the center of the formation. Armin Arlert watched as Luke Siss and Dita Ness attempted to kill it, cutting the tendons of its foot and making it fall, and successfully killing it. There is also an abnormal Titan which walks and runs on it is hands and feet and is capable of running at very high speeds. Its limbs are deformed giving it a spider-like gait and it is subdued by a captain leading it into a tree, knocking it unconscious.

Clash of the Titans arc
Connie Springer encounters his mother's Titan form in Ragako village. Its arms and legs were shrunken and paralyzed, and it said: "welcome home". It is later revealed that the Beast Titan is directly responsible for the humans-turned-Titans in the village.

When Mike Zacharias first encountered the Beast Titan, he believed it to be an abnormal Titan due to the mass amount of fur on its body.

After Eren and Ymir were kidnapped by Reiner and Bertolt, they stopped by in the Titan Forest between Wall Rose and Wall Maria. As they were resting atop the branches, an abnormal Titan was casually lying beneath them and appeared to be watching their every movement.

Royal Government arc
After drinking the serum that turns a person into a Titan, Rod Reiss turned into a gargantuan Titan twice as big as the Colossus Titan. He was so big that he could only crawl and his body generated enough heat to burn nearby trees.

This Titan turned out to be an abnormal Titan and it headed to the Orvud District, the nearest and most populated area, attracted by the amount of people in the city. It completely ignored the concentrated cannon fire against it and just kept going towards the city.

Trivia

 * The Nine Titans were considered to be abnormal Titans, but were classified differently.
 * Abnormals could vary in many ways from normal Titans. This included intelligence, such as that of the talking Titan, to immense sizes such as that of Rod Reiss's Titan, which stood at twice the height of the Colossus Titan.

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