Simply knock her out and quickly put items into her inventory.
Shadowmere has no encumbrance, so she can carry an infinite amount of items. Eventually, she will return to Fort Farragut if left unattended for a long period. She will stay wherever the Hero leaves her for a limited time. She will, however, die if she falls from a great height. Since she is essential, she cannot be truly killed in combat if her Health is reduced to 0, she will fall unconscious, only to revive later.
Her very high health and aggression make her valuable in combat. Shadowmere is the fastest horse available in Oblivion, and can climb inclines the Hero normally could not, making her helpful for navigating.
