Deathworm Morphant

Deathworms or Morphants were symbiotic creatures used to survive beyond death.

History
One account stated that Deathworms were created by the Morgs, however, another account noted that they were created on Skaro.

The Master used a Deathworm to survive his execution by the Daleks. (DWM: The Fallen) He managed to interfere with the Doctor's TARDIS to send it to the 30th December, 1999. With the form of the Deathworm, the Master was able to slither out of the TARDIS through 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 throat 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 Deathworm DNA, and the head scientist attempted to include the Deathworm'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 Deathworm's hunger. Transformed into a huge Deathworm and dwelling in London's sewers, he kept insisting he was in full control of himself the entire time. 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 Deathworm hybrid by slamming a combat helicopter on it. (DWM: The Fallen)

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