Nope, it's just really, really rare. It has a 2% chance of appearing on 1F and an 8% chance of appearing on B1F and B2F of the Chargestone Cave.

Is Liepard really all that bad? Mine seems to be performing rather well atm, though Assist will get less reliable when my team grows, and if i keep it on i'll have to get an Ice type instead of Haxorus to keep all the bases covered

Probably should ditch it just for balance purposes (Emboar, Elektross, Swanna, Simisage, Haxorus, and Krookodile is a lot more balanced than if Haxorus and Elektross were replaced completely by Cryogonal only to make room for another Dark type, regardless of how strong or weak that dark type is)...

Finally got one. Quick Ball FTW!

Liepard is probably the weakest Dark type, overall, in the games. Krookodile, Zoroark, Scragy, all perform far better than Liepard performs. 

And trust me, you do not want to pass on Haxorus, especially a Brave or Adamant nature one. The amount of Attack that it gets is completely insane. I went through the 7th and 8th gym using only Fraxure (on Pokemon Black) . That's how powerful it gets.

Right, my team is now currently as follows -

Pignite
Palpitoad
Solosis
Simisage
Tynamo
Tranquill

I'm in the Chargestone Cave, any tips on what to swap around or catch in the future?

Depending on whether you got the Plume Fossil, you would want to ditch Tranquill and start using Archen. Archeops is one of the best pokemon in the game, given it's very high speed, attack, Rock/Flying type and ability to use Acrobatics, which makes it a destructive force to be reckoned. 

Other than that, your team looks balanced, though personally, I prefer Litwick to either Solosis/Gothita. Chandelure is probably the best Sp.Atk of the game, and it's Ghost/Fire typing makes it very useful against two of the Elite Four.

I've just noticed that I missed the Desert Resort AND the Relic Castle... Fml.

Time to go back it seems

I get attached to these things, stupid as it may sound. Something hand-raised from a low-level holds more sentimental value than one caught at a higher value, though that feeling will pass

The fact that Krookodile learns a much better moveset (Crunch is the consummate Dark attack, then Foul Play and fricking Outrage) Haxorus being as strong as it is, and Elektross lacking any weakness seals the deal

I just realised too, that while fighting a Psychic type that I need a Dark type in my group. The Dark type I was planning on getting I can't get until Victory Road... 

Is it worth catching one and training it now, or just waiting for Deino?

To replace which one? You've got to factor in the peripheral uses of the Dark Types (Scraggy's Fighting or Krookodile's ground, etc) compared to what else you'll lose in the process