A Voyage Into Yesterday
I assume, from the tagline, that the monster on the cover is supposed to be some kind of dinosaur, but it sure looks like a dragon to me.
Later published as a paperback
Also includes H.P. Lovecraft's The Colour Out Of Space
This story was also published in A. Merritt's Fantasy
This story can be read in full at Project Gutenberg and scans of the actual pages of this issue are here.
The stupendous fantasy of the Aztec hounds turned loose on the modern world
With illustration, both by Virgil Finlay