Red Widow

The Red Widow, also known as the Spider Queen, is a malevolent species of all-female arachnid shapeshifters native to the Ravenloft setting of Dungeons & Dragons. This therianthrope breed falls under the category of "beastwere"; sapient giant spiders who have the ability to assume the forms of beautiful redhaired human (or humanoid, in 3e) women. In their true form, Red Widows appear as giant female black widow spiders with reversed coloration - red bodies with a black hourglass symbol on their underbelly.

Living a measly 20 to 30 years, Red Widows are usually little more ambitious than any other animal. Their food of necessity is blood; their food of choice is blood sucked from the horrified bodies of men lured into their embrace by the seductive beauty of their humanoid forms. Their usual MO is to get the sucker somewhere discrete with promises of a tryst, hold him tight, then revert to their spider form - aside from scaring the piss out of their victim, this also puts them in prime position to deliver a lethally venomous bite and commence to sucking blood.

To reproduce, Red Widows mate with humanoid men, then kill them and lay the eggs in their bodies to incubate - at least, in AD&D this is the case. 3e decided this wasn't grimdark enough; now the Red Widow conceives her offspring, paralyzes her lover with a special venom, and then implants her eggs into his body. This leaves him alive, aware and incapable of doing anything as his "daughters" hatch and proceed to slowly exsanguinate him from the inside out.

In AD&D, the Red Widow could summon 10d10 spiders (65% normal spiders, 20% large spiders, 10% huge spiders, 5% giant spiders) with no descriptor as to how often. These spiders adore the Red Widow and will gladly die to protect her, hence the "Spider Queen" alternative name. This ability was lost in 3rd edition.

As you can tell from all of this, whilst Red Widows fit the technical definition of monstergirls, being an all-female race of monsters with a seductive approach to interacting with human men, they subvert the spirit of the ideal by playing the role for horror and general "sex is evil; kill it on sight!" interactions.