"Sludge dumb" is how Grimlock, the Dinobot commander himself, describes the plodding Autobot. The Dinobots have a perhaps undeserved reputation for being dimwitted, but Sludge is the genuine article. He makes the other four look like scholars in comparison.
As Sludge can't make much of an impression in the mental department, he likes to make his presence known physically. He'll stomp his hefty hide around just so you know he's there. None of the Dinobots really dislike Optimus Prime, but he and Slag share an especially intense dislike for him. Sludge believes that the strongest should lead, and in his mind, Grimlock is the strongest Autobot. But Sludge genuinely believes in the Autobot cause, so he tolerates Prime. Beyond that, Sludge is actually what you might call nice... Maybe not to Swoop's level, but nicer than the other guys.
He is sometimes known as Slog.
His alt-mode is a robotic Apatosaurus