Between the publication of the first edition Monster Manual in 1977 and 1981 there were plenty of new additions to the monster roster for AD&D, some of those came out in the Dragon magazine Bestiary column, but many more came out in adventure modules which introduced new monsters like the Drow. Still other monsters came out in the British magazine White Dwarf, which at this time was mainly focused on AD&D, but which would soon become the house magazine for Games Workshop which would have their own games to sell, particularly a little one called Warhammer, both Fantasy and 40k. Idealized in 1979, which is the date on the Foreword, this would be repeatedly delayed, only coming out in mid-1981.

The Folio would bring together monsters from all these sources and as such become a kind of Monster Manual vol. 2, to be used in conjunction with the Manual to give DMs more options in terms of monsters. It does have, particularly because of the UK influence, a different feel to a lot of the Gygaxian creations for MM. While the MM covers a lot of the more obvious and traditional monsters of Fantasy, which makes sense for a first book compiling them, the Folio goes for more original and frankly weirder monsters. Some of them are kind of funny but others have become iconic.

The UK slant can be seen in the debut of some darker and more gritty monsters with complex and tragic histories, and no contributor did more for this kind of monster than Charles Stross who would have been the whole of 15 years old in 1979 and was already inventing the Death Knight, the Githyanki, Githzerai and Slaad, some of the more interesting monsters in the whole book. Stross would go on to be a Locus award winning and Hugo nominated science fiction writer. .