The kaijin(-monster) of the Grand Shadow (グランシャドウ) are all former humans.
They are transformed into this form through modification surgery.

Grand Shadow Kaijin List
Zinogre (1st executives)
Lindworm (2nd executives)
Blade Dragon (3rd executives)
Clayd Shark


[story]

Hey, what are you doing sneaking around on the beach at this time of night?
Oh, I'm not with the police or anything, don't worry... or should I say?

Oh, I didn't tell you my name. Sorry, sorry.
I'm Crayd-shark.
As you can see, I'm a Grand Shadow kaijin.

It's closed season in this town. You don't know that?
Well, I'm sure you knew that when you came here.
...I can smell the outsider in you.
Or, more accurately, a current that's not from this land. I'm a modified human with shark abilities. I can sense the slightest bioelectricity. I can also sense the presence of people like you...

Huh?　What are you reopening?
I want to praise you for your courage to stand up to me, but in order to protect the fishing industry, which is the foundation of this town, I can't allow people like you to get away with it.
That's how I repay my debt to the town I was born and raised in... Oh well. Anyway, just give up.

Hey, I'm not going to kill you. We didn't become monsters to do such things. We quit the human race with a certain ambition.

But you can talk about this all you want.
I just wonder how many of you poachers will take your stories seriously.

...Now, let's start the scary part.