Hi,
I am an extensive tester-user of the horsegift since it was implemented. I love the flavour it brings but I feel one tiny point still could be upgraded. Horse is closer to an inconvenience than a boon, which says it all.
While there were huge improvements over the years, there is still one part of it that makes it a dealbreaker for most : the micro management of the horse in transitions (interior and exterior, servers, etc) during adventuring and either the clunkyness of having it follow without dieing during a dungeon or abandonning it to die outside.
Why ? Because between the server swaps, the dungeons, all the interiors areas, the exterior areas that are treated like interior areas (ex : shadovar tradepost), the little "interior" transitions between exteriors (ex : brog docks to harpy nest), here how the normal adventure looks like :
1) Hero takes horse.
2) Rides proudly, (+rp) quickly, and efficiently to the first hub of his destination (via roads, boat, and many exterior areas). Note : Less and less true for epic levels where it's mostly indoor magical transportation. So your horse just isn't really an option.
3) Gets to the adventuring area where :
a) It's an all outside adventure. Rare, but presently as a rider I get to know them all. Plenty of those at low levels, few of them at mid levels, one or two at low epics, and then even fewer. From level 15 and on you are almost always doing the same areas if you don't want to burn everything in portal lenses and if you want to rp as a rider fighter, not just some guy that uses a horse "sometimes" to go from A to B. Doesn't sit well with the grind script.
b) It's mostly outside with some insides. Adventuring there with an horse is tedious and dangerous (might lose the horse on the interior parts or at transitions outside), but for the guy who really really wants to use his horsegift, it's still doable. Yet most of the time it will not contribute to a fun NOR immersive experience, and you end up not taking it at all with you. (Too much micro managing of the horse). And taking it to the entrance of a dungeon and leaving it there is both rp breaking (who in their right mind lets an horse attached to a tree in a monster area?), and a death sentence for the horse so then you must do for the fifteenth time the whole "my horse is dead" scene.
c) Half inside half outside, or more inside than outside : Taking the horse there would just be a masochistic endeavour. And generally leaving the horse outside "alone" is a death sentence so you end up not taking it at all.
So : A) You have fun with your horse, but you are ultra restricted in the areas you can go, AND you are not party friendly (because parties rightfully go to dungeons, not only the open areas).
B) After the novelty has passed you end up mostly not taking it anymore.
C) You don't take it.
At the end of the day when adventuring, you spend more time finding workarounds and ways to preserve your horse in a believable way rather than actually enjoying the experience of it. And well, just standing on the horse in town just to show off gets on people's nerves real fast.
To the suggestions now :
Suggestion A :
Make it a pokemon system like the keyrings. You can put your horse into some kind of "item" (saddle?) which then becomes summonable and unsummonable at will. If the horse dies, then you can't summon it until you pokeball another. Of course the summoned type would be the same as the pokeballed type (respects armors, colors, and flames *winkwink*).
Pros : Not clunky at all. Gameplay first. Makes horse totally usable to get somewhere, do the exterior parts on it, and then keep it safe but with you in the interior parts. In short perfect solution for "Gameplay/fun first" considerations and even, "interior immersion" as you won't have a stupid horse teleporting, getting in the way, fighting, getting killed stupidly, following you in the caves. Also solves some cross server issues (namely : Going from an interior place on a server, to another server, thinking of you shadovar tradepost, or the coast cavern with bandits east of cordor).
No balances issues.
Cons : For those that dont emote it, and for those with no imagination, it can look weird to have a guy summon or unsummon his horse. But actually now that i think about it, it's exactly the same as rangers summoning their petfriends without rping it as a magical summon. So ... not really something that could not already be a problem elsewhere.
Suggestion B :
Remove altogether any and every restriction on the "rideability" of horses. In short : ridable in interiors. Before everyone panics with the thought of "Mr. horsey" riding in Cordor's inn just to show off ... that won't happen, not a chance. Here's why : The guy who retired an epic character, got the luck to have a normal gift, and spent it for a freaking horse, don't want to ruin his involvment in the community, his ooc reputation, the character's reputation (which required the investment of a medium award!), and the DMs' trust, just to do a one time joke in Cordor's inn. It's not like the horsegift was easily accessible, people won't be stupid about it. And even if they are stupid about it, it can be handled ic like everything else.
Pros : Well you can ride in caves and dungeons, people might want you not to do it to avoid cluttering in corridors but that's also an ic problem. So the core problem is resolved : horsegift is perma usable no more workarounds, thousands of mounting and unmountings, losing your horse crossserver, etc ... you just ride your way to glory! And monsters do it, why not us?

Cons : Non cooperative horsegifters might be annoying to teams in dungeons. Tiny possible risk of exploit for archer+horsegift for kitting the foes (mostly dragons). But except for dragons, I doubt that, too many mages and archers, corridors and crossroads, that wouldn't be overly effective in interior areas. Risk of being stuck for very narrow areas (but then someone just has to unmount ...).
That's all for me for now. We're almost there on a fun and usable horsegift, I hope some incarnation of that suggestion will take place to lighten the mechanical micromanaging of the horse transitions.
Keep up the good work!
Gogo Neo-Triad!
ADDENDUM : Suggestion C (ultra conservative one) : Don't change a thing about horses, however mark areas that should or could be "exteriors" as ... exteriors. The ooc area you get to post reset, with two teleports (Gigashatten area or not sure how it's written), the shadovar tradepost, planes in general etc ....