A play on the term decepticon. In the prostitution industry, "transformer" means transvestite/transsexual, and "decepticon" means any very convincing transvestite/transsexual.
An "e-cepticon" is an online version, especially a female buddy or penpal whom you have known for many years, and who turns out (despite all evidence to tbe contrary) to actually be male.
HMB: I've been using this female username on this public game-related bulletin board for five years now, and people are pretty convinced I'm a woman, especially because I update the avatar occasionally with an female friend's picture. But, it's getting harder and harder to talk her into posing for avatar pics these days. I may just have to drop the whole charade and admit she was an e-cepticon.