Timeless Child's species

The Timeless Child's species were a largely unknown species from an unknown "realm", being, according to, the Doctor's original species as the Timeless Child.

Biology
They resembled Time Lords externally. However, this species was distinguished from the later Time Lords of Gallifrey by at least one feature: they naturally held the ability to regenerate into new bodies indefinitely, while the Time Lords needed to have an injection to splice the ability into their bodies. (TV: The Timeless Children)

Life cycle
When a member of this species was severely injured, their bodies would emit regeneration energy that could cause damage to the surrounding area.

When this happened, their appearance would change as well as their personality. The Master theorised that this race could regenerate an infinite amount of times. (TV: The Timeless Children) However the Thirteenth Doctor believed that the species possessed only twelve regenerations. (PROSE: The Secrets of the Timeless Child)

History
The Timeless Child, the only known member of this species, was found by the Shobogan explorer Tecteun near a monument on an unknown planet. The Child was stood beneath a wormhole (TV: The Timeless Children) to Universe Two. This universe was theorised by Tecteun to possibly be home to the Timeless Child's realm, although the Doctor was not so sure if that was the case. (TV: Survivors of the Flux)

After being brought to Gallifrey by Tecteun, the Timeless Child's biodata was used as a template to allow for regeneration in other Shobogans within the Citadel. Copying this ability had allowed them to become Time Lords, with noting that a little bit of the Doctor had been inside him all along. Unlike the Timeless Child's species, however, their copied version was limited to sets of twelve, though this appeared to be a decision made by Tecteun rather than a shortcoming. (TV: The Timeless Children)