Why are the monsters in the Witcher 3 look different than in the Witcher 1?

So I’ve noticed that a lot of monster had major design changes from the 1st game to the 3rd game. The most notable probably being the ghoul who went from a bipedal more zombie appearance to a quadrupedal fleshy abomination. Not that I don’t like the changes, I just want to know if there was a reason for the changes and if the previous appearances of these monsters still exist or not.