A zoan devil fruit is the kind of devil fruit that will transform you into an animal if you eat one. The fruit has three different transformations, Number one is the users normal form, second is a combination between the Users normal form, and the animal type, the final form makes the user transform completely into the type of animal that the devil fruit contains. The Zoan devil fruit has many advantages and disadvantages.
Advantages: include having abilities of the certain animal that the devil fruit gives you, increased strength and being able to mix the senses of your normal form and your animal form, and optimize them.
Disadvantages: things such as not having every ability of the animal, and the fact that if you are that type of devil fruit, for instance, if you eat the Inu-Inu no mi (mutt-mutt fruit) and you're a dog, well it will be useless to you, and you will never be able to swim ever again. Now normally a Zoan devil fruit will enable you three transformations, but Tony Tony Chopper of the Straw-hat Pirates has invented the Rumble Ball, which allows 5 extra transformations.