Specimen A was a Zillo Beast that had been cloned. Around 18 BBY, it managed to escape from where it was being held and proceeded to cause widespread destruction on the planet of Silla. Before its escape, this creature was confined within an Imperial scientific vessel; however, after breaking free and devouring the crew, the vessel ultimately crashed.
The original plan for "Specimen A" around 18 BBY involved keeping it contained on a transport until it could be delivered to a scientific facility situated in Mount Tantiss on the planet Wayland. However, an unexpected event occurred, leading to the Zillo Beast's breakout, during which it consumed all the crew members present. As a consequence of this incident, the transport plummeted onto the forest planet Silla.
When Clone Force 99 investigated the ship's wreckage, they encountered the beast in its juvenile stage. The beast absorbed the energy from the downed transport, rapidly increasing in size. It then fled toward the solar farm located near a village. Consuming even more energy at the farm, the Zillo Beast experienced further growth. Subsequently, Imperial forces arrived, capturing all the villagers who had witnessed the event, but failing to apprehend the elite group of clones. Specimen A was then returned to its initial destination for purposes that remain unknown.
At a later time, Omega came across the beast's aquatic enclosure, which was being guarded by TK stormtroopers. The Imperials were using ion cannons to subdue the creature. During her escape from Tantiss, Omega freed the beast with the intention of distracting the Imperial forces. The beast then went on a destructive rampage throughout the facility, killing two divisions of TK stormtroopers along with several technicians and scientists, before ultimately escaping into the jungles of Wayland.
The Star Wars: The Bad Batch episode titled "Metamorphosis," which premiered on March 1, 2023, featured Specimen A. The text "Specimen A" was displayed in Aurebesh on a monitor within the episode, identifying the Zillo Beast clone.