Lassoo

Lassoo is Mr. 4's gun-dog. Before it consumed a Devil Fruit through some unknown method which allows it to be a living being, it was nothing more than a gun.

Devil Fruit
The fruit gave the gun the ability to turn into a dachshund, becoming literally a dog-gun. In turn, Lassoo gains animal sentience and becomes a mobile mortar, increasing efficiency in combat. The fruit is known as the Mutt-Mutt Fruit in the 4Kids English dub and he is a mixed-breed dog, also called a mutt.

Cannonballs
On Mr. 4's call, Lassoo would cough up a seemingly infinite amount of exploding baseballs that (to Usopp's surprise) are not balls that explode on impact, but when a timer runs out. According to Chopper, Lassoo's baseballs are as heavy as cannonballs, thus ruining Chopper's plan to throw the ball back towards the agents before the baseball explodes. On the contrary, Mr. 4 can use his immense strength to strike these cannonballs with his 4-ton baseball bat, sending them flying at an incredible speed and accuracy at the opponents.

History
Lassoo teams up with the Mr. 4 and Miss Merry Christmas team to fight Usopp and Tony Tony Chopper.

Lassoo is ultimately defeated when Usopp tricks Mr. 4 into slugging Miss Merry Christmas with his 4-ton bat. While Mr. 4 was shocked at what had just happened, Usopp took advantage of this time to fire a hammer at Mr. 4's gut using Chopper's horns as a slingshot. Mr. 4 then flies into Lassoo and hits a stone structure thus knocking them both out, after which, Lassoo coughs up one last exploding baseball that supposedly finishes off the Mr. 4 team. However, the only thing it does is add more injury to the trio. Lassoo is then arrested by the Marines along with the rest of Baroque Works when Crocodile is defeated.

After all the members of Baroque Works have been exposed, Miss Goldenweek, Miss Valentine, and Mr. 5 fly to the Marine base and try to break out all of the captured members of Baroque Works. They succeed in rescuing Mr. 4, Miss Doublefinger, Miss Merry Christmas, and Lassoo (Crocodile and Mr. 1 decide to stay behind). Lassoo then helps out around the new Spiders Cafe as the new mascot.

Trivia

 * When Miss Goldenweek used her Rainbow Colour Trap on Lassoo, it is revealed that his dream is to become a tank. Later, when the released members of Baroque works open a new Spiders Cafe, Lassoo is shown in his new tank form instead of his old gun form.
 * When Mr. 4 calls for Lassoo, it is a reference to "Lassy".