Ultraman and Godzilla Monster Designs
Well it's really no surprise that Godzilla monster designs are similar to the monster of Ultraman designs. Reason? Here's why- Eiji Tsubaraya was the co-creator of Godzilla (while at Toho) while he became the main creator of Ultraman. Tsubaraya Productions became a special effects laboratory while Toho was also responsible with Ultraman. I mean you might want to take a look at most of Ultraman monsters are actually inspired by Godzilla monsters.
In fact, while Godzilla never appeared in any Ultraman series, however he was mentioned in Ultraman Tiga when an aleien went back in time to meet Eiji Tsubaraya (breaking the fourth wall). Also in Ultraman Max two children were playing with Gomess and Godzilla toys. And try checking the designs of Ultraman monsters, they are mostly very inspired by Godzilla monsters.