According to Alex Collier who claims to have received this info directly from the Pleiadians, the Reptoids were dumped into our Universe, because they were vile creatures which couldn't live with other species' in peace, and always on the warpath - manipulating, killing, pillaging, etc. They are not originally from our universe, although they mostly reside in the
Orion system supposedly, as well as one more system called
Alpha Draconis. Then there are some which are
interdimensional, residing in the 4th dimension, they do not have physical bodies - the only way they can interact with our dimension is through possession of humans which meet certain pre-requirements. Then there are also
hybrids, half-human half-reptilian; they are able to shapeshift into and maintain a human form like most of the examples I've presented so far, but sometimes lose their composure and their
"other features" begin to surface.
Here is the
original interview with Alex Collier from 1994, it'll answer your questions better than I can.
Concept drawings as well as some
answers about Draco Reptilians specifically.
That's a good example too, I forgot all about that video. It created so much buzz when it was first hosted by CNN, they had to pull it off their site shortly after.