Timeless Child's species

The Timeless Child's species were a largely unknown species from another universe or dimension, 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. (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)

History
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, seemingly having been dumped on a random planet near a monument.

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)

Behind the scenes
It is unclear whether this species has two hearts or if Gallifreyans do. If the former is true, then the Time Lords gained a second heart from the Timeless Child's DNA. If the latter is true, then the Doctor's second heart may be the result of experimentation by Tecteun and/or the Division.