This strange object on first impression looks like a lifeform, perhaps an eel or something of that nature. However, closer examination reveals a more artificial, manufactured look similar to a wrench or a spanner.
It is about 20 cm (8") in size, and was discovered by Rolf Varga in 2005 on Spirit Rover images Sols 527, 540 and 541. The Mars Archaeology Archive put together this GIF sequence to reveal the stuctural perspective of this very strange anomaly. A geological explanation might suggest a 'ventifact' - a piece of rock fashioned by wind-blown sand.
For more images and analysis: https://www.secretmars.com/maa012-tool-wrench