You can equip your creature with item cards. To form an item card, you must combine an equipment item and a blank unit card which you can purchase from a Merchant NPC.
Your creature must have learned the Unit Control rappelz-item.com/tag/skill/">skill in order to equip item cards. The higher the skill level of Unit Control is, the more item cards you can equip on a creature. Also, certain creature gets more bonus depending on which kind of equipment they wear.
To equip an item card, talk to the Tamer NPC and choose 'Creature Formation'. A familiar looking creature control window will open. Drag your item card onto an empty slot for your creature and it's done.
You can do just the opposite as well, by equipping the creature card on yourself! You must equip a belt in order to do so. Just drag the creature card onto your belt slot to equip. Different stat bonus is given depending on the creature Card skill.