User blog comment:OctoRift/Why The Cart Titan Should've Been a Regular Titan/@comment-1327106-20190211054157

When I first saw the Cart Titan during the Return to Shiganshina arc, I didn't think it was another person inside a Titan, like Reiner or Bertolt. I thought it was an unusually intelligent Titan, but otherwise a "regular" Titan. The reason for that was its design.

The Cart Titan looks so plain and not that much different from what we now know are pure Titans. And as you mention, there have been regular pure Titans that walk on all fours rather. The redesign with the elongated face helps make the Cart Titan a little more distinctive, but it still looks rather ordinary.

It's possible that Isayama did not have things fully planned out at the point that the quadrupedal Titan was introduced. So the fact there was a person inside it, and that it's one of the Nine Titans, could have been added post-introduction. He could have changed his mind about something, and the "fix" or the way he decided to make everything fit, was incorporating the talking quadruped into the Nine Titans.