The prop of Litra was modeled from the one of Rodan (called Radon in Japan) used in the 1964 Toho movie “Ghidorah, the Three-Headed Monster” based on the design drawn by Yasuyuki Inoue who was with the Toho Special Arts Division with real peacock feathers used for the tail.
The (re)modeling was done by Teizo Toshimitsu known as a sculptor of monsters who was with Toho in those days while the monster was intended to look like Phoenix.
Why such a tiny bird monster against Gomess then?
It seems to be because Toshihiro Iijima, director, thought a monster smaller than Gomess should draw more attention from kid viewers and make them cheer for Litra even more easily when he wrote the script under the pseudonym of Kitao Senzoku.
Incidentally, as the name Kitao Senzoku is often found in the production lists of the Ultra Series, it came from “Kita (north) Senzoku, the name of a place in Tokyo where Iijima used to live.
It looks like Eiji Tsuburaya told Hajime Tsuburaya, Eiji’s eldest son and director of the episode, not to deal with a bird monster anymore as Eiji knew the crew had to go through much trouble to shoot the scenes in which Largeus changed into a giant bird (it is said Eiji gave them a hand to shoot these scenes eventually) while the episode was shot before the Litra episode.
Eiji’s words, however, seem to have made Iijima even more obstinate about his decision to apply Litra to the Gomess episode.
I do love the easy-goingness of the pseudonym story and Iijima’s unyieldingness while these make me feel like such behind-the-scenes stories could reflect liveliness people had back then.