Crandios is a fossil pokemon. So expect it's rarity to be among the same as other fossil mons like Aero, Anorith, Lileep etc... So pretty rare, though biome would have an impact. They can be linked to a pokestop, I know this because I caught mine on one after seeing it in my nearly on a stop.
And regarding eggs, there have been no reports of any Crandios hatching froms eggs yet. If I had to guess an egg tier though, I'd say 10k (Because it's buddy distance is 5k).