Ground: Ratstallion. I like rats.
Flying: Grove warden.
It’s very beautiful, flies smoothly, isn’t too big, doesn’t have wings flapping. It’s perfect imo. When it was revealed, (and we didn’t know the source) I loved it so much I was ready to instantly buy it if it was from store. Unfortunately you can’t get it anymore, but I’d be ok with it returning, so that more could people get it too. Such a good mount.
This. Yak is the best ground mount. Not my favourite, but the best.
For me it’s the Reins of the Headless Horseman.
Took me years of running the holiday dungeon to get and finally got it 3 years ago.
This is a great mount. I love how it matches most of my mogs.
But on the other hand, I really hate being pigeonholed into using a handful of mounts.
My favorite mount is me.
Nothing very rare, I guess: Fallen Charger (ground mount from SL) and Smoldering Ember Wyrm (flying mount from Legion, I think). I normally do the “summon random favorite mount” thing though.
Probably the heart breaker from the event
All the foxes. 
Sapphire Panther, because it’s small and works as both.
The JC panthers are the coolest mounts in the game. So underrated.
#showtooltip
/castsequence Netherlord's Brimstone Wrathsteed, Netherlord's Chaotic Wrathsteed,Netherlord's Accursed Wrathsteed,Headless Horseman's Mount
For pure ground mount I like the mawsworn charger you get from Torghast. Like its shadowy form under all the plate. A runner up is the fel charger warlock mount. Looks especially cool once you get the green fire effect.
For legacy flying I like the headless horseman’s mount. Lucked out and got it the first time offered and it’s been my go to. Very Warlocky without all the extra spikes.
For dragon riding I like the wylderdrake. The others are just too big for my tastes.
My troll shaman is an engineer and use a self-built gyrocopter when flying non dragon-riding.
Lucky break. I’m playing a warlock so I get the felsteed horse at level 20. It was just a matter of visiting my warlock trainer. No more bouncing along on the Blood Elf chicken.
Happyness is galluping through the Silverpine forest on my felsteed.
Brutosaur. And it was acquired with a lot of hard working repeatedly clicking through payment confirmations.
What was ol’ Blanchy sins? and Why is he running from them?
My favorite is Shackled Ur’Zul from Antorous
Long ago in Escape from Durnholde Blanchy alerted the Tarren Mill guards to Thrall because she was jealous of Bessy, a reincarnation of Ainur, the pantheon on the Second Ones, thereby being an accessory to the Infinite Dragonflight thereby enemy of the Titans thereby actually a void lord disguised as a horse who is actually a Dark First One and it wasn’t the soul of Argus but Blanchy who knocked the Arbiter out in CATA and crash landed in Revendreth and now she was stuck in an infinite time loop until we “saved” her but in reality we’ve released her.
This is a conspiracy I’ll put the tin foil hat on for.
The mount I used for several expansions, was the white wolf from AV.
For flying mounts, I really liked the purple drake from yearly achievement (What a strange long trip).
Those kinds of mounts are fun. Working towards them, not this 0.01% chance rate.
So many great choices. My favourites are the slime cat and breath of Ohn’ahra. The former just because I love cat mounts, and will always cycle through those, and I’m still defiantly using slow flying until my beautiful bird can truly soar. It’s just a really beautiful bird model.