Around 18 BBY, "Specimen A" was held in a transport and was originally planned to be sent to a scientific facility in Mount Tantiss, on the planet Wayland. Due to an unforeseen incident, the Zillo Beast escaped its captivity and ate the crew aboard. The event caused the transport to crash on the nearby forest planet of Silla.
The beast was discovered in a juvenile form by Clone Force 99 when they went to explore the wreckage of the ship. The beast consumed the energy of the transport and grew larger shortly before escaping and making its way toward the solar farm of a nearby village. The Zillo Beast quickly consumed even more energy from the farm until it grew to a much larger size. Imperial forces then came to retrieve the giant beast and detained all villagers who bore witness to it but were unable to capture the elite squad of clones. Specimen A was soon sent back to its original destination for further unknown purposes.
Some time later, Omega discovered the beast's watery enclosure, which was guarded by TK stormtroopers. The Imperials used ion cannons to keep the beast tamed. During her escape from Tantiss, Omega released the creature to divert the attention of Imperial units. The beast subsequently rampaged through the facility, eliminating two divisions of TK stormtroopers and several technicians and scientists before escaping into the jungles of Wayland.
Specimen A appeared in the Star Wars: The Bad Batch episode "Metamorphosis," which released on March 1, 2023. The Zillo Beast clone was identified as "Specimen A" in Aurebesh text displayed on a monitor in the episode.