Phobos is one of the two satellites (moons) of Mars, Deimos his neighbor is a bit smaller.
As you probably know the asteroid belt is between Mars and Jupiter and we think that Phobos was an asteroid from this belt captured by the gravity of Mars and has become one of its moons.
It has a very small size and is not a sphere which is another reason for us to assume that it was an asteroid.
Diameter:10km
Discovered by: Asaph Hall
Discovery date: August 18, 1877