Clockwise from upper-left: "Phoenix Beast Gairuda", "Ironhammer the Giant", "D.D. Esper Star Sparrow" and "Beast-Warrior Puma".
These monsters are based on characters from Nelson's television show "The Sparrow" ("D.D. ESPer Star Robin" in the Japanese version). The main monster is "D.D. Esper Star Sparrow" as it also represents its owner. They are geared towards Summoning "D.D. Jet Iron".
These monsters and Nelson's television show "The Sparrow" seem to reference a 1971 manga series named Babel II. Both feature a junior high school student turned alien hero and three servants, a black cat servant, a flying servant and a loyal humanoid robot. It also features an evil monarch relative conspiring to take over the world, similar to "Galaxy Queen".