Yes, most people think Endlers are similar or are wild guppies but they've been given a separate species name and have been classified different from guppies as of yet.
In regards to this particular fish being a hybrid; I've been keeping Endlers off and on for over 8 years and I've never ever seen an pure Endler (class N) with that much black on the tail. There are quite a few hobbyist throughout that don't care for the classes (read about it here if interested:
http://endlers-usa.com/) that aren't as strict about keeping their bloodlines pure and freely cross their Endlers (Poecilia wingei) with plain/fancy guppies (Poecilia reticulata) which would produce a hybrid (Class K). My guess (and simply a guess) is that whoever was the original source for this user's particular Endler, possibly crossed a half black guppy with a Endler.