A hybrid of obesa would be called Euphorbia obesa x. The other order would refer to a hybrid with its own signifier, for example this natural Dudleya hybrid.
Dudleya x semiteres
We do not have database pages for unnamed, unnatural hybrids (like greenhouse hybrids of E. obesa) as a matter of policy. I would think there is some concern about the number of entries multiplying, and those extra entries being confusing because the hybrids could theoretically be with hundreds of plants in the genus, so the page could be filled with plants that look very unlike each other because they only share 50% of their genes in common.
I realize there are a few database pages for species where some of the plants are hybrids. I have struggled to deal with some of them and sort out the confusion. For example, this page for E. milii has a number of E. x lomi hybrids and presumably other hybrids of milii on it:
Crown of Thorns (Euphorbia milii)
and while we do have an entry for E. x lomi, I'm not confident I could identify and separate all the plants on the milii page above which would fall into that category, or even be distinct from the species itself. Here is the separate entry for E. x lomi (which was a natural hybrid before it was popular as a greenhouse hybrid):
Crown of Thorns (Euphorbia x lomi)
I suspect some of the hybrid milii cultivars are not even milii x lophogona but instead hybrids crossed with species or other hybrids, which makes identification a nightmare.
I agree with you about the two Echinopsis images above. That's another genus where hybrids tend to pop up under the name of species. There are other examples in the database. We could move them to the genus entry and probably should, but I'm not really up to the task of contacting all those different people and checking on each and every plant. I moderate the Euphorbias (and a few other succulent genera, but not Echinopsis) so that is where I have decided to focus my efforts.
I hope that makes sense.
If at any time you think an image should be moved, by all means start a thread for that image and we can set the wheels in motion.
