If one type of Monster Girl could be real, I'd hope it would be the Slime.
Simply because of how immensely distinct and evolutionarily distant Suu is compared to every other complex living organism.
Has none of the things we would normally associate with a sapient beings.
At least most the other MGs, there is at least some shared commonality in form.
Slime has none of it.
She may as well be alien.
And yet, she is able to meet or exceed all we ourselves are capable. Its kind of mind boggling.