Starting out with the new toon, everything was normal at first. I used the horseshoe to get an 'Old Nag'. Used her for a bit, then while trying to access the cargo later, there was a 'poof', and the chat filled with folks complaining about disappearing mounts. I tried getting on the horse, but game wouldn't let me. So, went to the garage and towed it back. At that time, there were several 'extra' horses in the garage that also indicated they needed to be towed.
In any event, the garage slots were magically filled to the 5 max. I had Old Nag, Horse (that I had also crafted), Running Horse, Racing Horse, and Wasteland Runner. None of the new horses had any Cargo Space at all. Zero, Zip, Nada. Not sure if they're supposed to or not. I was going to try crafting a Riding Horse with its 16 slots, but then Clinton F.A.R.M. went down, and I could no longer log any characters in there.
Sounds like the garages are filling with random mounts. Some get ATVs, some get Motorbikes, etc... I just happened to get all horse mounts. *shrugs*
Regards.