Elaina St. James may be a cougar ... but she's also an OnlyFans model who just achieved a historic landmark in the sex-toy game -- and Lamar Odom may want to add her new product to his expanding collection.

The 57-year-old St. James -- who also goes by "America's Favorite Cougar" -- has scored one heck of an achievement ... she's become the oldest model ever to collaborate with Fleshlight!

As you know -- or maybe you don't -- Fleshlight is a popular sex toy brand that creates artificial vaginas and other imitation sex devices -- including oral and anal. St. James has just inspired the company to make a replica of her lady bits --- and we even got her to pose with the private part made out of a secret material.

St. James -- who's also an author and podcaster -- tells TMZ her collaboration with one of the top pleasure-device brands for men in the U.S. is "surreal" at her age, especially when she thinks back 5 years ago when she was barely making ends meet to pay for her son's braces.

Elaina also tells us her down-to-earth personality mixed with her humble background as a working mom captivates her followers ... while defying stereotypes around age, sexuality and reinvention.

This is where OnlyFans comes in ... a few years ago, she says she started her journey with the subscription-based platform, and she's become a star attraction. She says she wants her success to remind women there's never an expiration date on sexiness and sensuality.