Bananawani

Bananawani (バナナワニ, English versions: Bananagators) are giant crocodiles with banana-shaped growths on their heads. They are so fierce that they even prey on Sea Kings and are to date the Sea Kings only known natural predator. Despite their strength they were easily defeated by Sanji. They are classified as type "C" creatures being "Big Savage". They first appeared in Chapter 127 and Episode 77

Crocodile had many Bananawani as pets in his old (now destroyed) casino in Rainbase. He could control them as they were often seen in the room with him and Nico Robin without any sort of aggressive attitude toward either of them. Crocodile seems to have had them well-trained as they would also follow his commands, such as eating Mr. 3 and the key he dropped into their den merely by looking at one. Their stomach acid is resisted by Mr. 3's wax.

Bananawani is a portmanteau of the Japanese word "wani" (which means alligator or crocodile) and the English word "banana."

F-Wani
The F-Wani (F-ワニ, Accellegator in the English manga) is a type of Bananawani that is less ferocious than his counterparts and goes very fast is the Sandora Desert. It lives in Arabasta. Nico Robin once uses this type of animal (in chapter 180).

Trivia

 * Bananawani are the first species that are fiercer than Sea Kings owned by a Shichibukai (Crocodile), the second being Yuda owned by Boa Hancock. Both of them also represent their owner's reptilian animal themes, crocodiles and snakes respectively.