The Bureau of Land Management is being challenged over its plans to remove the horses from five herd management areas known as the Calico complex, leaving 600-900 behind.
If it goes ahead, it will be one of the biggest musters in years.
The bureau has plans to remove 12,000 horses from the western rangelands this fiscal year, adding to the more than 32,000 already held in federal facilities.
The challenge to the Calico muster is being mounted by the group, In Defense of Animals, and wildlife ecologist Craig Downer.
The bureau says the muster is required because forage and water supplies have become limited due to drought in the area. It says the horses' winter range is limited and inadequate resources exist to support the current population.
It argues the reduction in herd numbers will help achieve a thriving natural ecological balance.
However, opponents argue the muster is unnecessary, as well as cruel and dangerous. They argue the bureau has failed to follow proper legal processes in its plans for the muster.
US District Judge Paul Friedman heard the arguments last week.
If the judge rules in the bureau's favour, the muster is scheduled to go ahead on December 28. It is likely to take three months and has an estimated cost of $US1.7 million.