Thanks - I'm really okay with either interpretation (yours/SRD and/or Dr. Zombie's) - I imagine that under your interpretation, GP, that when Hiritus was done riding for ten hours, Justice would stop, Hiritus would just know to dismount, and then Justice would wander away, to reappear at dawn or whenever they set out.
Or, under Dr. Z's, Hiritus has to maintain a commitment to this horse, come hell or highwater, and like he says, if we're in level 4 of the Temple of Very Bad Things, Justice wouldn't appear or disappear. In fact, Justice would have gotten them to the door, then hung out to "supervise" the other horses, help to protect them from wild animals, keep them from needing to be tied up, thus eating a perfect circle into the grass for the duration of the crawl, etc. That works for me, too, and in fact, it would help establish commonality with the other "permanent" horses such as Vesper, which I like.
As for setting, and plausibility, I would tend to play it like I did this time: To the untrained observer, the horse would come walking from somewhere "off camera." Sort of mystical, fade in, fade out-ish, like Zorro, but holier.

Not -boom! flash! horse!- like I envision Aerda's summoning (which, by the way, isn't
mount a cantrip?). Anyway, I think this works for the flavor of the game and the plot so far, both.
Either way, I've never heard of a paladin having to pay for the horse he called in any way, and the material from everybody seems to agree. And in any case, Hiritus shelled out for the military saddle, tack, studded leather barding and saddlebags regardless - believe it.
Which leaves him in range (with the turn in of the recruitment armor, and Fendric's additional help) to upgrade to proper paladin armor. Studded leather's for rookies, and rogues.
So off I go to get that worked out...
Thanks, y'all. I appreciate the help.