The Mercurians are a humanoid species that developed an advanced civilisation on the first planet of the Solar System. Not much is known about Mercurian biology, but their body appears to be made to endure temperatures as high as 700 Kelvin (430 degrees Celsius). As a side effect, they seem to be quite vulnerable to the effects of colder ones and will resort to special suits allowing them to keep their heat when travelling off world.
Men of Mercury action figures where commercialised in the sixties. Their story was told at the back of their blisterpack:
On Mercury, the planet nearest to the Sun, great rivers of molten lava churn and flow past majestic cities, glowing red hot in the blinding radiance of the Sun. Here, in tempeature so great that life as we know it would be consumed in a flash, the mighty volcan people live - beings of living fire that burns eternally. Protected by suits of thermal alloy, lest the freezing cold of outer space would snuff them out, INFERNO and the Men of Mercury travel the universe in the great chariots of fire that we call comets.