Good question, this tweaking is more common than I thought -
I would say many of my monsters are tweaked. perhaps as high as 60%, my reasoning is the the variety of things all the monster manuals is just to great to inhabit a world. So I have normal lizard men, LM the the aboleth have tinkerd with (skum) and gaint lizard men (ogres)
So it feels like there is only a few creature types, with gentic offshoots the 1000s of monsters with no place in biology, or ecology.