Loners is the nickname for some black or blue creatures in Avacyn Restored which get better when they're the only creature a player controls. They represent the thematic opposite of the pairs created by Soulbond. The "Loner"-mechanic turned out be unpopular, and didn't return for Shadows over Innistrad. According to Mark Rosewater, it is extremely unlikely to return.
Certain aura enchantments only work for loners:
While other global enchantments requires you to have only one creature to take effect