Pluto may be a KBO also. Further, it's possible some moons of the gas-giant planets are captured KBO's.
The unusual thing about Ultima Thule is that it's in a nearly circular and "flat" orbit, which many experts interpret to mean it's mostly undisturbed from its point of origin. It's a prime "fossil".
Pluto has a "disturbed" orbit such that its origin is currently unknown. Same for gas-giant moons. Pluto has also been turned inside-out, perhaps multiple times, by a still unidentified force. Ultima Thule is probably mostly as-is since formation.
Pluto may be a KBO also. Further, it's possible some moons of the gas-giant planets are captured KBO's.
The unusual thing about Ultima Thule is that it's in a nearly circular and "flat" orbit, which many experts interpret to mean it's mostly undisturbed from its point of origin. It's a prime "fossil".
Pluto has a "disturbed" orbit such that its origin is currently unknown. Same for gas-giant moons. Pluto has also been turned inside-out, perhaps multiple times, by a still unidentified force. Ultima Thule is probably mostly as-is since formation.