Lagoon Monster Mask.
Beasts being born of the unknown it's not surprising that there are a lot of water creatures in the Beasts! book. I haven't counted but considering how little most of us get in or on the water it's disproportionately large... unless you consider the basic symbolism of water, and the trend of these creatures being suddenly born from it only to retreat back into its incubating depths. In any case, here we have an ad from the perennial vessel of popular monster culture, "Famous Monsters" Magazine.