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An icy small solar system body having a tail is a comet.
A small solar system body (SSSB) is neither a planet, nor a satellite. All other objects which circle the sun are called SSSBs. Comets are the most common SSSBs and they are very cold on their surface. As they get warmer (when they come close to the Sun), they begin to release gases which form a tail behind them. This process is called outgassing.
Comets can sometimes be seen from the Earth without the need of a telescope and can be as big as 30 kms in radii. One of the most famous comets is the Halley comet which revolves around the Sun in 76 years. It was last seen in 1986.
The surface of a comet is very cold and is a collection of small rocky particles, gasses and ice. There are a total of around 6500 known comets orbiting the Sun as of 2019.
