DungeonsAndMobs is the definitive solution to create custom mobs and items. It lets you create custom mobs in a full-customizable GUI (you can change item's material, display name, lore, slot and even function) and custom items from a simple YAML file.
Be aware that D&M is as user friendly as possible, but it has a lot of features, and an unexperienced user could find it complicated. Contact me for any problem, question or just to set it up.
Graphics by WildLori
Before buying read carefully everything, especially the "Requirements" sections below.