I'm trying to find if this info is somewhere in one of the books, but have had no luck.
We know that it is Notatus that made the breaktrough for Aegis of the Hearth.
Do we have a similar information (i.e. its original creator) for Hermes Portal somewhere? Is it "simply" a Mercurian ritual that was "hermeticized"? Or was it part of the stable of spells from the Terrae-magi of Guernicus' tradition?
No such info provided AFAIK. Most spells do not have a listed creator and IIRC this is the case here.
Xavi
I haven't seen it either, which is why I asked. Plus the fact that I don't have all the books, so it could have been mentioned in one I don't have!
I believe that the True Lineage books claims that it was Mercere prior to his loss of magic that invented the Hermes Portal enchantments. (no info on the spell itself, just the portals)
This seemed strange to me since his background was Fertility Magic.. but there you go. Which is why Harco (Mercere HQ) has the most portals. but there you go.
It does indeed claim that he was the inventor, and moreover made the Mercuarie gates...
However this does not seem to fit with his magics being 'weak' (Infact in my mind a maga must have been less powerful at the founding of the order before the great spread and sharing of knowldge, so I think of the founders as strong in a specilised area, but less great then current arch-maga) - so I think in my saga he will have physically brought the ancient text containing the basis of the ritual - from which it is later be incorporated into hermetic theory... possibly by one of his apprentices (who would then also go on the set up the gates at Harco.) 
The Founders have the reputation of being amongst the most powerful wizards of their day. It sounds like Mercere really lived up to this reputation.
Though he may have specialised in enchantments rather than flashy spell casting. I don't think even Verditus could turn a ritual spell like Hermes Portal into an enchanted item.
I can half imagine that the first project the newly minted Hermetic Founders engaged in was the group development of the Hermes Portal ritual. Sort of a proof of concept that this new fangled magic works to bring together disparate wizards.
I initially thought HP came from Guernicus or one of his apprentices, since his was a strong Terram lineage. But there is nothing about that in the House's description. Since the spell lists "Mecrutian Ritual" as the base, and Guernicus did collect many rituals from the tradition, this may simply have been translated to Hermetic parameters from one of those.