The S is for Sucks Dragon
From Homestar Runner Wiki
Strong Bad's first attempt at drawing a dragon made in the email dragon was a failed one, thus he added a caption reading The S is for Sucks to the picture. According to Strong Bad, it doesn't look natural. Its body is made from an S and a more-different S, and it has beefy arms and legs and a triangle-shaped head. The more popular Trogdor retains a single beefy arm coming out of the back of his neck (for good measure), a similarly-shaped body, and three spikes on his lower back.
On the fifth anniversary of the email, Strong Bad celebrated The S is for Sucks Dragon (instead of Trogdor) with both a children's alphabet song and a heavy metal song. The heavy-metal-backed drawings at the end of the Trogday 08 cartoon portray The S is for Sucks as an aquatic type of dragon, or sea serpent, a designation likely facilitated by its serpentine body shape. This is in contrast to its counterpart, Trogdor, who is more of a "wing-a-ling" dragon. The song also revealed that The S is for Sucks's teeth, spinities and angry eyebrows are drawn using "constabulate V's", rather than the "consummate V's" of Trogdor.