My personal suggestions are:
Land:
Rillaboom
Appletun
Dracozolt
Arctozolt
Dracovish
Zacian
Zamazenta
Water:
Intellion
Drednaw
Dracovish
Arctovish
Dragapult
Flying:
Intellion
Corviknight
Flapple
Dragapult
Eternatus
There are certainly more that could be mountable, but the ones above are just off the top of my head. Any other Gen 8 mount suggestions?