That's actually exactly why it was asked. Many people are using the OGL to put out 5e products
If you are asking for yourself, my advice to you would be to buy as many OGL bestiaries as possible, and learn what is in them, so you can 1) borrow whatever you like, and 2) make sure you're not reinventing the wheel. When in doubt, check one of the srds, such as the aforementioned d20pfsrd.com and see what they have included.
If you are asking on behalf of publishers, most of them have freelancers, I imagine, who are already doing the above.
If you are asking as a thought exercise, carry-on, but the list of closed monsters is really much, much shorter than the list of open monsters. DLIMedia already gave the list of closed monsters. I'll repeat them and offer a few thoughts...
Beholder (and gauth) - As I mentioned once, there are variants of the beholder that are open, most notably the aquatic version, eye of the deep. But beholders are one of those monsters that can be easily done without if you have to.
Carrion Crawler I actually find this one the most annoying to be closed. Again, as mentioned before, the juvenile version and a moth version are both in Tome of Horrors.
Displacer Beast This is one of the more dubious closed monsters, as the displacer beast is a ripoff of the Coeurl. But check out the Pathfinder Kamadan.
Umber Hulk There's a weaker form of this in Nemoren's Vault. You can also use a gray render for a similar feel.
Githyanki & Githzerai These are easily replaced. Try a denizen of leng, or a tiefling.
Slaad (all types) Easily replaced with the Pathfinder protean.
Mind Flayer The intellect devourer and the aboleth are both open and can easily take the place, story-wise, of the illithid. Or use the eldritch spawn template from Book of Monster Templates. (If you do the latter, let me know, as I will be tickled).
Kuo-Toa Use Sahuagin, or deep ones
Yuan-ti (all types) The yaun-ti are a ripoff of serpent-folk. Serpent folk are open. Use the original instead of the imitation.