Lassoo

Lassoo is Mr. 4's gun-dog. At some point earlier in the timeline, it consumed a Devil Fruit through some unknown method which allows it to be a living being, though before, it was nothing more than a gun.

Appearance
Lassoo is a sandy-brown gun, similar to a bazooka, that has handles on the sides and a contoured barrel. Lassoo is always suffering from some kind of cold that makes him have a running nose and sneeze (which makes him fire cannonballs). It is unknown if this is an actual disease or a side-effect from being an inanimate gun unnaturally absorbing a devil-fruit.

Personality
He seems to be very loyal to his master, as he did everything he said without question.

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 as he is a mixed-breed dog, also called a mutt. During battle, Lassoo functions as an automatic baseball pitching machine of sorts for his master to hit the baseball ammunition with towards their targets.

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.

Alabasta Arc
Lassoo teamed 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, Lasso coughs up one last exploding baseball that supposedly finishes off the Mr. 4 team. However, the only thing it does is further injure the trio. Lassoo is then arrested by the Marines along with the rest of Baroque Works when Crocodile is defeated.

Miss Goldenweek's "Operation: Meet Baroque Works"
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.

Major Battles
Lassoo, Mr. 4 and Miss Merry Christmas vs. Chopper and Usopp

Trivia

 * When Miss Goldenweek used her Rainbow Color Trap on Lassoo, it is revealed that his dream is to become a tank. Later, when the released members of Baroque works opened a new Spiders Cafe, Lasso is shown in his new tank form instead of his old gun form.
 * The name of a gun is M4. This is perhaps a word game with the name of "Mr. 4" is that Lassoo is a gun. This could also be a reference to the M4 "Sherman" tank.