Every time Butt Stallion is fed, one of the following will happen:

Butt Stallion will flatulate, producing either a green weapon, or rarely a health, ammo, or money pickup.
Butt Stallion will vomit a rainbow and either a blue weapon, or rarely a health, ammo, or money pickup.
Butt Stallion will rear up, whinny, and excrete a purple weapon. These have a high, though not guaranteed, chance to use the special Gemstone material. If a Gemstone weapon is produced, it has a much higher chance of being a Pistol than any other weapon type. Gemstone material is not available for Rocket Launchers.
Due to a separate bug, a Hyperion E-tech Dart or Spiker may be produced instead of a purple gun, possibly with a gemstone skin. Hyperion Darts and Spikers are the only E-tech weapons which Butt Stallion can produce, and the only E-techs which can appear with a gemstone skin.
Non-Gemstone weapons produced by Butt Stallion will be thrown a shorter distance than Gemstone weapons.
Butt Stallion will whinny, prance in place and turn 180 degrees, then excrete a legendary item.