Monsters
Monsters live on mana more than food—they'll survive on mana, but thrive with both food and mana.
Over time if they don't get mana, they'll essentially become rabid and start to grow much more aggressive and bestial. It's a painful process, which is why they're more aggressive.
This also explains why monsters outside the dungeon have to be cared for a specific way. Barns (including the guild's) emit mana inside, much like the dungeon but through artificial means.
The guild also trades in items that can be used to provide mana, the most common of which being collars so that normal people without barns can have a monster. There are other appearances these items can have, too: earrings, necklaces, etc, but the collar is most popular because the monsters are pets, essentially.