The ASN courier droid, also designated as the ASN-121 assassin/sentry droid, or more simply the ASN-121 assassin droid, represented a specific model within the courier droid category. A modified ASN unit, known as ASN-121, was deployed by the bounty hunter Zam Wesell during her attempt to assassinate Senator Padmé Amidala. Wesell programmed this droid to deliver a pair of poisonous kouhuns into Amidala's quarters, but the Jedi guardian of the Senator, Anakin Skywalker, killed the creatures. Obi-Wan Kenobi, Skywalker's mentor, then latched onto the droid, hoping it would lead him back to its owner; however, before the courier could find Wesell, she used her sniper rifle to destroy it.
This droid model continued to see use throughout the galaxy for decades following the conclusion of the Clone Wars, with some even finding employment in Imperial service on Murkhana. Crimson Dawn provided Ochi with an ASN unit, specifically ASN-909, with the purpose of assassinating members of the Emperor's Royal Guard using a poison known as Ziroxin-12.
The initial appearance of ASN courier droids occurred in the 2002 Star Wars film, considered canon, titled Star Wars: Episode II Attack of the Clones. Their formal identification took place within the 2015 reference book Star Wars: Absolutely Everything You Need to Know, authored by Adam Bray, Kerrie Dougherty, Cole Horton, and Michael Kogge.
While Absolutely Everything You Need to Know listed the ASN courier droid's height as 0.3 meters, the 2020 reference book Star Wars: Extraordinary Droids by Simon Beecroft indicated their height to be 0.38 meters.