Creatures are species without a lust meter. They are immune to seduction, but also unable to seduce. They cannot work and do not require a salary, but still require food. They can't equip items, but gain greater stats in exchange. Normal creatures can be fused or bred with their own kind, or fused with a spirit of any kind. Spirit creatures follow the spirit fusion rules. Most spirits are creatures, but spirit creatures are listed under the spirits category.
Recruiting a creature in the wild requires a net consumable item or a net skill (found on spider girls and a variety of spirits). Recruitment depends on level difference and creature's current health.
They can also be tamed in the forest near the city.
On some days, there's a special trader present at the market who will buy creatures from you. Offered price depends on the type (base cost), size (+50% per tier), and level (+5% per level) of the creature. Protip: catch some Magic Deer on those days.
Note: There is a randomly encountered NPC in the Portals (Seliel) who can give you a choice between a bird, wolf, and a mule.