GRENDIZER: GETTER ROBO G - GREAT MAZINGER KESSEN! DAIKAIJUU

A giant sea monster known as "Dragonsaurus" has emerged from out of nowhere, terrorizing the depths of the oceans. Great Mazinger, Grendizer and the Getter Robo G team join forces to combat the new threat. But their task becomes more complicated when Dragonsaurus swallows up Boss Borot and makes its way towards Tokyo. (Source: Anime News Network) — Description from AniList community (anilist.co).