The Roman Candles are sapient, energy-based life forms native to the planet Mercury.
In their natural shape, they appear as colorful balls of light and absorb energy directly from the sun. However, they're able to assume an illusory physical appearance based on images they gather from the minds of Human settlers - as a result appearing as an imperfect representation of whatever the subject is thinking. Still, in five hundred years of colonization, they've never actually communicated with Humans. They're also speculated to be immortal.
- "Masquerade", by Clifford D. Simak (1941)