I think the mega-able and shiny mega-able things show up only when
you're hunting a pokemon that can be a mega able. To use your
ecamples: Vulpix (alolan or otherwise) doesn't have a mega
evolution, thus it can't hatch mega able and therefore having the
mega icons show up on the shiny hunt would be pointless since the
count would always be 0. Shinx, on the other hand, has a mega
evolution on PH so the mega options are there. Does that make
sense? :,D
The wiki is handy indeed but it doesn't seem to be completely up to
date though. Autumn mareep is missing from the event list and the
page doesn't yet list the new PH exclusive mega pokemon, which
are:
> Mega Milotic
> Mega Flygon
> Mega Claydol
> Mega Luxray
> Mega Froslass (which has been around quite a bit longer)
Giratina can also mega evolve on PH but it works a bit differently
than other megas. Any giratina can mega evolve when given the item
Griseous Pearl to hold.