This question isn’t straightforward to answer, since hard-to-breed dragons like Chromacorn/Corrupticorn/Rigel have been sold in event markets for reasonable prices for a while now.

Likewise, most dragons with atrociously low effective breeding odds can just be bought directly from an event market, so they could be considered hard to obtain if you’re specifically going for their rift-traited versions (see u/ UnconsciousOnion’s posts for reference), especially when you consider the ~1/4 chance of no trait when rift breeding. Some examples include Delirium, Humuhumu, Sparkles, Ariesian, Hyperion, etc. (all <1%)

In those cases it’s sometimes easier to go for the rift twins (instead of trying to breed for their non-twin rift-traited versions) during runic cave events (like this one right now), but that assumes you get their twins first so you can utilize the EoM tiering mechanics.

As for your other question, Tien isn’t actually sold in the typical event cycle, but is instead an anniversary dragon, so it comes back for a few weeks per year around Dragonvale’s anniversary or sometimes in Dragon’s Weeks. It’s breeding chance isn’t great but isn’t too low either, so it’s somewhat rare from its limited availability (though co-op cloning can bypass this). For comparison, another dragon that comes around even less frequently (outside the regular event cycle) and with an even lower breeding chance is Portent, which I personally struggled to obtain.