Although these anthologies in the Beaver imprint were aimed at younger readers, the stories used to fill them are certainly not children's stories - surprisingly far from it in some cases!
Slime by Joseph Payne Brennan (1953). An attempt at Lovecraftian horror by someone who seems to misunderstand what Lovecraft was doing and thinks that a murderous rampage is the same as cosmic dread. The weakest story in the volume.
Running Wolf by Algernon Blackwood (1920). Blackwood takes us to the untamed reaches of Canada, as used so effectively in his classic 'The Wendigo'. This is a slight story, but told very well.
The Terror of Blue John Gap by Arthur Conan Doyle (1910). Fun story about the monster in the abandoned mine. Doyle cleverly adds verisimilitude through some unexplained details.
The Troll by T. H. White (1935). This story is crazy. It has gore, it has sex, it has madness. It would surely have melted the brain of any youngster reading this collection. Easily the best story in here, and one that would stand repeated and close reading.
The Fog Horn by Ray Bradbury (1951). The evocative tones of a coastal fog horn summons a 'Loch Ness' style survivor of prehistoric earth. The idea is better than the telling.
The Quest for Blank Cleveringi by Patricia Highsmith (1967). A botanist is determined to discover and name a species of giant snail. A very silly story that is still great fun to read.
The Lonesome Place by August Derleth (1948). When Derleth drops the Lovecraft pastiches and writes with his own voice, he can be really good. This story is really good! The horrors that only a child knows take terrible form.
The Valley of the Beasts by Algernon Blackwood (1921). A man who has an unfortunate tendancy to act like a beast becomes more beast than he bargained on. A nice clever little story.
It by Theodore Sturgeon (1940). Probably the scariest story in the collection, 'It' is an unfeeling, unexplained monster from the swamp who kills simply to try to understand.