I would say maybe 70 - 75% are second edition versions of 1E critters.
I tried my best to not re-include what I call "character beasts", or unique monsters from very specific adventures that could not possibly appear elsewhere. It takes too much from the old books and does not leave any big surprises.
Now, some of the big baddies - frost worms, man-serpent (god in the bowl) and the monstrosities from Hell (Cthuloids) still HAD to be included...they are just too cool not to get a new coat of paint.
But be sure that ALL the animals and non-unique demons will be back, many of which with newly sharpened teeth and fangs!
Cheers,
Bry