In fanart, they are often depicted with sharp teeth, and a lot of his AU selves have sharp teeth. What's interesting is that many of the monster characters have sharp teeth, but Sans and Papyrus do not in canon.
He is part of the "monster race" in Undertale. Sans has a much more detailed description on the Undertale Wiki. The Undertale fandom was known to be disproportionately horny for nearly all the characters, but Sans' position as the fan favorite saw him as the most prominent subject of that horniness. The dramatic tension of the scene skyrocketed Sans into being the fan favorite, so much so that he eventually represented Undertale in Super Smash Bros. In this battle, Sans has a far more aggressive and involved attitude than in the rest of the game and reveals a surprising amount of raw combat power. This battle is often considered the most difficult boss battle in the game. In the game's Neutral or Pacifist routes, the player will never fight him, whereas in the game's Genocide (or No Mercy) route, Sans is the final significant boss. His role in Undertale's story depends on the route taken. He has a brother, Papyrus, whose energetic and genuine personality contrasts Sans' lazy jokester personality. Sans is a skeleton, and the first character the player meets outside of the Ruins. 2.6 Fanon Splintering and Self-Shipping.