Tohl Narita writes in his art book that he designed Bostang almost just like a stingray shown in the photos he liked.
He also explains that the episode to feature Clapton (not the guitarist) was cancelled for reasons related to the shooting and that the kaiju ended up being switched to a stingray monster along with the episode replaced by ‘Space Directive M774.’
That makes me imagine the episode featuring Clapton could require miniatures of oil plants and it might have been the reason why it was left unproduced (or I also feel like I may have read so somewhere before).
As to Peter, he states he drew it just like Chameleon as it was adding that, whereas he made it a rule not to design mere giant versions of real-life creatures, he also felt he should be in no rush recollecting Peter should have been the first or second monster he designed.
Regarding Sudar, a monstrous fish-like creature was properly designed by Narita for the episode ‘Fury of the South Sea,’ but he says in the art book that the design was not used as the episode was plotted to feature a giant octopus from the very beginning.
The design Narita describes as an experimental design of a monstrous fish seems to have been the piece he liked, and he writes it would have become a representative monster if it should have been realized.
As described above, it should be good to remember it’s not that he always got to create a monster as he liked according to the intentions of the production members.