Kalub's creature (Lords and Masters)
Talk about it here.
When Doctor Kalub worked at the Kyme Institute on time travel, he had genetically engineered a living creature with a biological time and space travel capability. Kalub then imprisoned the creature within a stasis field and harnessed the time energies it gave off as a perpetual source of power. The stasis fields held it tightly and caused the creature pain, though it continued to try to escape unaware that it gave off more energy as it struggled to escape its prison, continuing the cycle.
Roughly humanoid, the creature was trapped in a bubble of energy while it writhing as though it were drowning in water with its arms and legs kicking out. Its limbs phased in and out of existence so it was solid one moment and then smoke the next as it strained people's perception of it.
Its body shifted as well, in age and gender so it was both old and young, a bone white skeleton and a cluster of cells while shifting from man, woman and child, to a corpse as it fluctuated in eternal agony.
The Time Lords became aware of the creature when they foresaw a time where the stasis fields gave out and it caused damage to history. They couldn't take direct action because they were still trying to hide their presence from the universe after the Time War, so they sent Missy and Yayani to deal with it. Yayani killed Kalub, but Missy took the creature and jettisoned her Eye of Harmony and used it as a power source instead.
When Missy placed the creature in her TARDIS, she imprisoned it in a stronger containment field which would never break, not caring she was condemning a life-form to perpetual torment as it tried to escape from her TARDIS. (PROSE: Lords and Masters)