Mothra Leo is a character that appears not in a Godzilla movie, but a TV series, Godzilla Island. He is referred to as "Little Mothra", also on a season with the same name.
If you are wondering about Leo's origin, thinking that he stared in the Rebirth of Mothra trilogy click here
In Godzilla Island, Mothra Leo is referred to as Little Mothra (子モスラ Ko Mosura) as a larva, but becomes known as New Mothra (新モスラ Shin Mosura) after maturing into his imago stage.
The larval Mothra Leo was with his dying mother inside of a cave when a meteor crashed, which turned out to be Hedorah. Mothra, while still in bad condition, flew towards Hedorah and fought him, but Hedorah brought the aging Mothra down.
Hedorah then went after Leo, but Mothra got back up and picked him up, then with her remaining strength flew to a volcano and dropped him into it. Now struggling to fly, Mothra fell into the volcano too.
Leo then left and formed a cocoon. Hedorah turned out to be alive when he came back out of the volcano. Hedorah saw Leo's cocoon and went after it, but was distracted by Godzilla. Leo emerged from the cocoon in his imago stage, then teamed up with Godzilla to defeat him.
Infrared rays - Mothra Leo can also fire red beams from his antennae.
Leo was knocked out of the sky by the SpaceGodzilla-possessed Godzilla's atomic breath. He was also defeated in battle by Mecha-King Ghidorah and Kamacuras.