Where i can catch them?
Darkling is a reward for having thirty lvl 99 arks. Spinoheat you get from eggs though it may be a future OM though I cant be sure. Neither are catchable in the wild.
Yep Spinoheat is egg only and Darkling is the pre evolution of Necrodrake
When will new arks be released
I refer you to my monster list (part 2). This covers those monsters (and you can type CTRL + F or CMD + F and type in Darkling or Spinoheat to jump to them).
This is for future reference, though your question has been answered already.