Anthills generally contain ammo, reagents, general weapons/armor, and a small percentage chance to roll something rare similar to chests. The items that drop seem to vary based on the level of the anthill. A lvl 20 Anthill will drop different rare loot than a lvl 25 Anthill.
Known Drops:
Lvl 15-19:
Lvl 20-24:
Massive Scale Heater
Lvl 25-29:
Lvl 30-34:
Lvl 35:39: