I’ve been able to track down almost every mount in D4. There’s a total of 99 mounts in the game that are ownable by players (doesn’t mean obtainable) including MTX, horses and cats. Not including datamined alpha test mounts. I was able to obtain 98/99 mounts in D4 (including all cat mounts) and there’s one left in the game that I am missing which is the ‘reins of the decaying steed’. I’ve never seen it drop nor do I know anyone who has seen it drop but I believe it is obtainable in game as a rare drop from treasure Goblins. Has anyone found this mount yet? let me know.
Yea I have that one, literally looks like a horse from resident evil or something, flesh is falling off the bones. Had no idea how rare it was.
https://imgur.com/a/dfqtXHf
https://imgur.com/a/T59IXAy
For some odd reason I can’t post pictures again, must of done something the CM’s didn’t like.
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I have it too. I only have 40 or so mounts though.
Yup they don’t even tell you “why” anymore.
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Total of 35 mounts apparently myself. You can tell I’m not much of a collector lol.
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Only 32 for me but I do not purchase anything from the shop nor BP. I have him. 
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I also “own” one.
The only mounts i like so far are the ghostly horsy and the cat mounts from VoH.
My GF is searching since the introduction for the mount, that can be dropped dy the walking dead WB.
The only i’ll always have is my faithfull Ol’Nell…
I don’t consciously collect them. I certainly don’t buy any. I just pick them up when I run across them in the game. Surprisingly I have 47 mounts including ‘reins of the decaying steed’
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43 mounts, one being the Ugliest Possible of Horses. Also the only thing I’ve ever purchased from The Shop are skimpy armor for attractive Classes so basically I’ve never purchased anything from The Shop.
I acquired it somewhere between S0 - S2 from a treasure goblin.
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I have everything that I can obtain from the game and buy from the Shop. There are a few exceptions because I just don’t like them, for example Delights of Agony ( I don’t like that they slashed the horse the way they did), Fleshrot Parasite (YUCK! JUST YUCK!) and maybe a couple more that I refuse to buy coz they’re gross, they’re not “cool” gross, just gross! 
I’ve used all of them except the Reins of the Longfur Beast, the mount I got from completing The Dark Citadel for reasons I’ve mentioned in previous posts.
There are mounts I’ve never obtained for one reason or another:
Brigand’s (Random Anywhere)
Cavalier (RA)
Dark (RA)
Executioner’s (RA)
Grisly Calico (Unknown)
Plains Yearling (Unk)
Seal Brown (RA)
Taiga Roan (Random @ Fractured Peaks)
Trapper (Unk)
I think some of the mounts I listed above might not have been included at launch nor in the expansion, no idea really…
Those mounts are Alpha test mounts. They were datamined in 0.0 but they’re not in the game. They were prototype mounts used by the devs to test the horse system, some were also early drafts of mounts that either didn’t make the cut or were redesigned/renamed only to be added later on in a MTX shop update.