From the Super Mario Wiki, the Mario encyclopedia
Mechakoopa is Bowser Jr.'s down special move in Super Smash Bros. for Nintendo 3DS / Wii U and Super Smash Bros. Ultimate. The Junior Clown Car ejects a Mechakoopa, which walks across the ground, turning around if it hits a ledge or a wall, and explodes after some time. If it runs into any opponents, it hits them, leaps into the air, and explodes. The Mechakoopa can also be picked up and thrown, which causes it to explode on contact. Only one Mechakoopa can be deployed at a time.
Bowser Jr. has two additional variants of this special move in Super Smash Bros. for Nintendo 3DS / Wii U:
The first variation is Impatient Mechakoopa, where Bowser Jr. flings a Mechakoopa from his Junior Clown Car. This Mechakoopa will immediately explode upon touching ground, but the explosion deals less damage than normal. The Mechakoopa explodes prematurely if it hits any opponents before landing.
The second variation is Big Mechakoopa, where Bowser Jr. deploys a large Mechakoopa from his Junior Clown Car. The large Mechakoopa walks forward at a slow pace before self-destructing, but unlike the default Mechakoopa, the large Mechakoopa can only deal damage when it explodes. Fighters can pick up and throw the large Mechakoopa, but it will self-destruct if the fighter holds onto it for too long. Lastly, the large Mechakoopa's explosion deals more damage and knockback than normal.