Axos is an entity that came to Earth, supposedly in need of fuel. Its real motivations were to drain all energy from Earth in an attempt to gain the secret of time travel...
Biology
Axos was composed of several parts, but the most noticeable parts were the Axonite and the Axons.
Axonite
Axonite was the miracle element that the Axons traded with Mr. Chinn for the energy to re-fuel their ship. Axonite was a living substance and they claimed that it could be used to enlarge the size of organisms. It was sent around the world to key scientific points, where it would drain energy evenly from those points. (DW: The Claws of Axos)
Axons
Though Axons had humanoid features, they too were part of the main Axos structure. They first took the form of golden skinned humanoids, but they could also take the form of large, tentacled creatures. They were capable of morphing between various humanoid shapes and teleporting themselves and others. (DW: The Claws of Axos)
Strengths and Weaknesses
All the elements of Axos are unharmed by gunfire. Axos the craft however can be stunned if a certain tube is shot. All Elements of Axos can feel the pain of each other. Axons are easily destroyed by explosions.
History
Axos encountered the Master at an unknown point in time. They captured his TARDIS and kept him prisoner; he directed them toward Earth.
Following their arrival on Earth, they attempted to start a 'feeding cycle'. They met with UNIT and promised them large amounts of axonite so they could enlarge food and end world hunger. UNIT ordered worldwide distribution of Axos. The Doctor realised that the Axons planned to use the axonite on Earth to drain the planet of all its energy. The Doctor stopped them by placing Axos in a time loop. (DW: The Claws of Axos)
The Axons managed to escape the time loop, and infiltrated 21st Century Japan. Disguising themselves as humans, they produced a Television programme for children that promoted a new kind of soft drink, Goruda. When the children drank the liquid, they gained telpathic abilities and were slowly transformed into Axons. (DWM: The Golden Ones)
An Axon was imprisoned by the Department and imprisoned in Dauntless Prison, it was later set free by K9 Mark 2 and his friends. (K9TV: Liberation)
As it was wearing Human clothing, it may have came from the Japan invasion.