Timeless Child's species

The Timeless Child's species were a largely unknown species from another planet, universe or dimension, being the Doctor's original species as the Timeless Child. They were extremely similar to Time Lords physiologically, as the Doctor was able to spend their known life under the belief that they were a Time Lord.

However, this species differentiated themselves from the Time Lords by at least one factor: they naturally held the ability to regenerate into new bodies indefinitely. The Timeless Child, the only known member of this species, was found near a boundary to another reality or dimension, from which they had come through.

After being brought to Gallifrey by the Shobogan 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)

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)