Ouroboros
Ouroboros - also spelt "Ourobouros" - is usually represented as a dragon biting its own tail. This dragon is best known from Greek and Norse mythology but also appears in other traditions. The name means "tail swallower".
The serpent eating its own tail has many important symbolic connotations. It can be seen as representing the unity as well as its cyclic nature. It nourishes itself by consuming itself, it is forever changing whilst always the same.
The Ouroboros symbol plays an important role in alchemy, both as a symbol of the cyclic universe and as a symbol of purification. A double Ouroboros - two creatures swallowing each other - is said to stand for volatility.
Ouroboros has also been connected with similar traditions in other cultures such as Abraxas in Persion mythology and the Norse world serpent Jormungandr.
More recently, August Kekule is said to have been inspired to discover the structure of benzene by a dream of Ouroboros.