While the monsters in the Monster Manuals are primarily authored by Gary Gygax, the monsters in the Fiend Folio are created by players of the game itself in the United Kingdom. In truth, the book is a compilation of other people's creations, and some monsters (namely from the D series) from AD&D modules.
That said, Fiend Folio feature some monsters that survives to this day in the 3rd edition: the slaad, kenku, githyanki, githzerai, aarakocra, achaierai, bullywug, the ever popular death knight, mephits, gibberlings, grimlocks, penaggalan, to name a few.
Not to mention there aren't stinkers...there are a whole bunch of monsters of questionable worth, like the flumph, an unsually aligned (lawful good) thing that was once voted the most useless monster in AD&D, the gambado, the garbug (wasp bodies lobsters(!), forst men, tirapheg, umpleby, adherers, just to name a few.
There are also a few standouts like the Elemental Princes of Evil, and the nilbog (goblin spelt backward).
One big downer however, is the art. Although there are a few by old time favs like Jeff Dee and Erol Otus, but too many are by other artists whose style are too violently graphic and lack the fantasy feel of the first monster manual.
Overall, Fiend Folio is good, but not as good as Monster Manual.