Fire Ant Gatherers located just north of Muspelheim entrance from Vale of Mularn do not leash. Under any circumstance. If you can maintain just enough speed to stay ahead of them, they'll follow you into eternity, or until you zone/die.
These mobs are part of the Muspelheim Explorer: Fire Ants (20-24) quest line. I have tried it with the Fire Ant Scavangers or Fire Ant Workers, so they may or may not do the same.