>>8783761A lot of it has to do with the Marvel Godzilla design, but back in the day it seems like companies that did licensed merch/promo material for various media and IPs had a lot more artistic freedom and apparently didn't have to adhere to any super-specific guidelines like today (see pic related).
I'm sure a lot of designers and artists took a look at Godzilla and said "oh he's supposed to be a T-Rex with spines" and just went with that.