Going directly from the Avatar wiki entry on the Opening Sequence, the silhouette is Sud, Avatar Roku's earthbending teacher.
The sequence opens by showcasing the four elemental bending styles: waterbending, earthbending, firebending, and airbending. The waterbending is performed by Pakku, the earthbending by Sud, the firebending by Azula, and the airbending by an Air Nomad.
From Sud's wiki entry:
Of the four shadowed benders featured in the opening sequence, Sud was the third, and currently last, to have been identified, after Pakku and Azula.
The reasoning for this is as follows:
Sud's character design was confirmed by Michael Dante DiMartino and Bryan Konietzko to be a prototype for Toph Beifong.
Toon Zone News Interviews Bryan Konietzko & Mike DiMartino
From Toph's Wiki entry under Original Design:
This idea made its way into the opening sequence, as the shadowed earthbender was confirmed by the creators to be Toph's "prototype" and in the opening sequence of the unaired pilot episode."
And on The Boulder's wiki entry, under Trivia:
He bore a resemblance to Sud, Roku's earthbending teacher, who was modeled after an early design of Toph.