For ages, the dragon has meant chaos, and the most primal force of nature. In the Middle Ages, it may have been seen as evil. These days, it’s just a fantasy trope.
Tiamat was possibly the original dragon (or dragonness) of chaos, dating all the way back to Sumeria (though I personally tend to picture her as a beautiful goddess). The idea would also appear in Canaan, in the form of the monster Yamm or Lotan. And in the Bible, it would appear in the form of Leviathan. Similar is the Greek monster known as Typhon, and the Indian Vritra.
Leviathan also features in the Satanic pentagram, in the form of the Hebraic letters on each point of the pentagram. But way before the Church of Satan was founded, a dragon or serpent is used in the circle in the Goetia. It only fits that a dragon (Ouroboros) sometimes appears on the pentagram, or that Leviathan is spelled on the pentagram, because in the Christianity, Satan himself is sometimes referred to as a dragon.
The dragon is associated with power, and as I mentioned, chaos, carnal forces, the most primal force of nature, and often the path of the sorcerer. Funny enough, a dragon is associated with the god Marduk or Bel.
All this makes the dragon a perfect symbol for me.