How do I fix a mustang that panics when separated from other horses?

Separation anxiety in a mustang — the horse panicking, calling, refusing to work, or becoming dangerous when taken away from companion horses — is a specific management and training challenge that reflects both the horse's evolutionary dependence on herd membership for safety and the specific vulnerability of a horse whose primary social security in captivity is other horses rather than a deep enough relationship with its handler to find security in the human's company. A mustang with strong herd-bound behavior has typically not developed the specific type of trust in its handler that provides the psychological security to manage separation from companions — the horse relies on other horses for safety rather than on the handler, which means increasing the depth and quality of the horse-handler relationship is as important as any specific separation training. The most effective correction approach builds the horse's tolerance for separation gradually — beginning with very brief, low-distance separations where the companion is visible, then building to longer durations and greater distances as the horse confirms it can manage each level calmly. Returning to the companion when the horse is calm rather than when it is panicking is the critical training principle, because returning during panic teaches the horse that panicking produces reunion while returning during calm teaches the horse that calm produces return. The handler relationship development that runs alongside separation training includes spending non-demanding time with the horse — grazing together, quiet grooming, simply being present — that builds the horse's association of the handler as a source of positive experience and psychological security rather than purely a source of training demands. A horse that genuinely looks to its handler for leadership and safety tolerates separation from companions significantly better than one whose relationship with the handler is primarily about compliance under training pressure.