To clarify, when I say "summoned creatures not made from spells", I mean summons that are from class features and the like, so the Shadow Sorcerer's Hound of Ill Omen, the Hexblade's Accursed Specter, Raven Queen's Sentinel Raven (tho less hp on this would be nice) and the Beastmaster/Drakewarden's Animal/Drake Companion. Sinces these are features that summon creatures, would they work with the Shepherd Druid's Mighty Summoner if they aren't technically spells?
Mighty summoner does not apply to non-spell summonings
The relevant line from the class feature (XGtE, p. 24; emphasis added):
Any beast or fey summoned or created by a spell that you cast gains the following benefits:
The feature is missing any clause, such as "... or other feature/magic" which would have the effects apply to creatures summoned by features.
As a side note on the listed examples; Hound of Ill Omen would not work because it is also a monstrosity, the Accursed Specter is undead, and the Drakewarden is a dragon.