Morphant

A morphant was a shape-shifting, snake-like creature native to Skaro.

History
The Master used a morphant to survive his execution by the Daleks. (DWM: The Fallen) He managed to interferre in The Doctor's TARDIS to send it to the 30th December 1999. With the form of the Morphant, the Master was able to slither out of the TARDIS via its keyhole and across the pavement, the Doctor saw this but he was unable to warn Chang Lee. Later that night the Master infiltrated the house of Bruce, the paramedic who attended to the wounded Doctor. He slithered down his mouth while he was asleep and possessed him, gaining a new body. (DW: Doctor Who)

Using a sample of the substance the Master had spat at her, Grace Holloway initiated an ambitious project intending to imbue humans with regenerative abilities, believing it to be Time Lord DNA. Unfortunately, as it happened, it was Morphant DNA, and the head scientist attempted to include the Morphant's abilities into his own genetic makeup. As he lacked the mental powers of the Master, he was unable to retain his own identity and was left a slave to the Morphant's hunger, transformed into a huge Morphant dwelling in London's sewers, though he kept insisting he was in full control of himself the entire time. Instead, he started eating people and absorbing their knowledge. Izzy Sinclair nearly fell prey to its cannibalistic desires, though upon detecting she had contact with the Doctor, she was left alive as a bargaining chip. The Doctor destroyed the Morphant hybrid by slamming a combat helicopter on it. (DWM: The Fallen)

Behind the Scenes
The creature was never named on-screen, only in the script and comic story. According to The Eight Doctors, the creature the Master used was a deathworm.