There are three catchable Mythicals and one Secret
The database lists Kitsune ($31/sec), Lightning Dragon ($34/sec) and Red Panda ($29/sec) as Mythical, plus Christmas Dragon ($42/sec) as Secret. Everything above that — Galaxy Kitsune, Galaxy Axolotl, Cosmic Griffin, Celestial Griffin, Celestial Hydra — is breed-only and cannot be caught at all.
That split defines the endgame: three catches and one seasonal event feed a breeding tree that produces everything else.
Red Panda first, always
Red Panda is the outlier of the Mythical tier: Hard rather than Hardcore difficulty, and it only needs Rain. No overlapping conditions, no event gate. At $29/sec it out-earns every Legendary except nothing — it beats all of them.
It is also the parent for both Galaxy recipes (with Kitsune, and with Axolotl), so a spare Red Panda is never wasted. If you own one Mythical, this should be it.
Kitsune: catch it or breed it
Catching Kitsune requires Night and Rain to overlap in the deep forest. That overlap is not guaranteed to occur while you are online, which makes the spawn unreliable regardless of your skill.
The breeding route is Moon Cat + Yeti at 12%, needing Night and Blizzard. Moon Cat is the easiest Legendary to catch at night and Yeti is farmable on any Blizzard, so the recipe is repeatable in a way the spawn is not. Most players end up breeding their first Kitsune.
Lightning Dragon: the gear check
Lightning Dragon only spawns during Thunderstorm in the volcanic zone, and it is the clearest gear check in the game. Below Cosmic Chain strength (420) the strike phase snaps the rope mid-catch, so attempts on a Nebula Lasso mostly fail.
It is worth the trouble twice over: $34/sec is the highest farmable income, and it parents both Hydra (with Cerberus, 8%, Thunderstorm) and Cosmic Griffin (with Christmas Dragon, 6%, Blizzard + Event Charm).
Christmas Dragon and the event problem
At $42/sec, Christmas Dragon is the top of the income table, and it only spawns during its seasonal Blizzard event. Outside that event there is no way to obtain one — no code, no recipe, no trade path documented.
If you own one, it is also the only route into Cosmic Griffin, and therefore into Celestial Griffin and Celestial Hydra. Do not spend it casually.
The Celestial ceiling
Celestial Hydra is the rarest pet in the game at a 5% roll, and both of its parents are themselves bred: Hydra at 8% and Cosmic Griffin at 6%. Multiply those odds and add the Legendary+ Breeding Station and a Thunderstorm requirement, and you have a target measured in weeks, not sessions.
The realistic endgame for most accounts is a pen of bred Mythicals — Red Panda, Kitsune, Galaxy Kitsune — plus a farmed Lightning Dragon. Treat Celestials as a long project rather than a plan.