Well, in addition to giving this a bump for Mercule's question, I'll ask one myself...
Recently, the SRD was updated and made entirely official. However, some creatures/races were removed and retained, presumably as Product Identity (i.e., no 3rd Party can use them short of special permissions, and word is that some have already obtained the "green light").
Might I ask, for the sake of curiosity, where the ideas for the following originated?
-Beholder
-Carrion Crawler
-Displacer Beast
-Kuo-toa
-Mind Flayer
-Slaad
-Umber Hulk
-Yuan-ti
I'm not trying to snub WotC, mind you. I'm just trying to figure out what makes these more PI than, say, owlbears and ropers.