kapuchu said
Considering that Theá has (most likely) attempted it more than a few times, I would say that an offspring between mortal and immortal is impossible. Granted, it may just be that the chance of success is infinitely small.
Since alicorns are somewhat based on the idea of greek gods, I don't really see a reason why they couldn't have children with each other or mortals. It was the kind of thing that happened all the time. The thing about demi gods is that despite having divine blood in them (And no doubt having some neat tricks to go with it) they are still mortal and thus grow old and die sooner or later. Add on that the abilities tend to get passed on by the bloodline (Even if they might grow weaker with each successive generation) and its quite possible for Thea to have a X to the power of Great grandchild around somewhere.
Now that I think about it, there is a cannon precedent for this theory in the form of the ex Prince Blueblood,
Nephew of Princess Celestia. Considering how many years it has been since an alicorn other than Luna and Celestia has been to the mortal plane and working under the assumption that the benefits of divine blood start to wane over successive generations, it is completely possible that Prince Blueblood was an arrogantly proud descendant of one of our characters from a coupling hundreds, if not thousands of years ago.
To quote the grand Faust herself:
Lauren Faust stated that Celestia's nephew had no name in the show (before The Best Night Ever aired), and that he's the "great great great great great great great great great great great (and probably even more greats) nephew on Celestia's and Luna's mother's side, about 52 times removed, roughly speaking." He was originally a Duke, but was changed to a Prince because "someone in the approval process thought kids wouldn't know what a Duke is",[3] though the title of Duke would eventually be used for other ponies much later.