Japan has had a lot of these "special" evolution materials just for certain monsters, but usually it is for collaborations. I'd assume this is a similar deal vs Tamadras since these materials are just for Zeus and not for the majority of monsters like Tamadras. We'll probably get a new Zeus Descended Dungeon to show off his new ultimate forms, and these monsters will appear as enemies in it. Drop/appearance rate wise I'd assume they would be on the level of those yellow cats in the War Dragon dungeons, especially if the new dungeon is Mystical level.
Still, they are going to have to do a lot with Zeus' ultimates to make it worth the effort of hunting them down, especially with so many good monster options now for clearing most of the game's content (both in and outside the Rare Egg Machine).
I was reading that saying to myself, "I would never have suggested mono-light, so I need to edit that pronto."
