How to make a customized hitbox?

 Hello everyone, I would like to ask if it's possible to make a customized hitbox. Since the default hitbox form is of a cube, this is bad for making mobs like lizards, fishes, etc, in general, mobs that has a size of X greater than Z and vice versa. See these mobs I've made, they're 3 bosses, one is a huge Lisard with characteristics of a Spinossaurus, his hitbox should be more like a parallelepiped, so it would cover all the model and leave empty what there's no model parts. but istead of this, the hitbox just make pushes the player away because he try to go on its side, or behind it but the hitbox is in this "empty" places and doens't let the player even approaches. is there a way to modify the hitbox form? Also, the "view" (blue line that is aways in front of the head of a mob) can be moved?