UPDATE: Also check out the more recent post on these figures for better photos and additional information.
What: In addition to the Space Creatures and Space Raiders lines, Diener Industries produced a lesser-known collection of one-color plastic and rubber monster figures based on mythology. Sculpts include Medusa, a minotaur, a centaur, a unicorn, a dragon (basilisk?) and a birdman (roc?). As with the Space Creatures, I have seen versions made in both a harder plastic as well as a softer eraser-like rubber.
When: While I got these figures in the late 1990s, I'd guess they were first produced alongside the Space Creatures, in the 1970s-1980s.
Where: U.S. (at least). Sold in dime/variety stores and probably other places.
Who: Diener Industries, Inc.