Gigan the inspiration for dnd Hook Horros?

 I think so.

Gigan appeared in 1972. 
Monster Manual I appeared in 1977. 

I mean, a birdlike beak and head plus hooks for hands. It's super similar, I can't be the only one who thought so. 

1e MM Hook Horror. 

3e version even more so... check out the belly spikes. The longer hooked arms. Yeah?

1983 Dungeons and Dragons cartoon episode "City at the Edge of Midnight"

Some nice references: 

https://bloodwork13.blogspot.com/2016/03/kaijuology-gigan.html

https://www.youtube.com/watch?v=mmHMyFmqn60&ab_channel=TorenAtkinson

https://www.youtube.com/watch?v=i_IBbrrL-v4&t=2s&ab_channel=Dungeons%26DragonsTV

https://www.youtube.com/watch?v=VLm6QuC2xdA&ab_channel=TorenAtkinson

https://saveversusallwands.blogspot.com/2017/11/phantom-stalker-spectral-hound-and.html