>>57169687I would imagine some combination of:
The mythology of the Ryujin and wyverns/dragons and birds (the former has a legend that has parallels with Lugia's being a dragon that is the god of the sea and dragons have parallels with Phoenixes in Eastern culture, Lugia's general body plan seems to be some mixture of draconian and aviary traits).
I think the softer aspects of it's design (having "hands" at the end of it's wings, a round belly and being characterized as a "guardian", being guardian of the sea, staying away from people so as to not hurt them and being a parent despite being a Legendary in the anime, being the first case of Legendaries being shown to reproduce in any capacity) is due to Shudo's desire for it to have Maternal traits as well as the sea being the mother of all life in many peoples' mind but that obviously got sidelined hard what with the male VA, lack of any water typing and such.
It does make somewhat whale-like noises in the first movie too, so many a Whale of some kind?
Honestly, it's hard to say, which is the beauty of early Pokemon, the designs being vague enough that you can't quite say for certain what a Pokemon is is half the fun.
Like good ol Nidoran (F or M) they start out as some sort of uni-bunny thing and end up as uni-bunny/humanoid/somewhat reptilian mini-Kaijus.
