Quick answer: Frostallion Noct breeds from two Frostallion Noct, or from a Helzephyr + Frostallion pairing.
Frostallion Noct is the Dark-type variant of Frostallion. Unlike most Legendaries, it has a genuine alternate route: pairing a Helzephyr with a standard Frostallion also produces it, alongside the expected same-species pairing.
Step by step
Route A: obtain two Frostallion Noct and breed them directly.
Route B: pair one Helzephyr with one Frostallion instead — no need to own a Frostallion Noct at all.
Either way, breed at a Breeding Farm (base level 19) with Cake stocked.