Taken from another post from Graffe's
In order to get the models off advantages, you need to KEEP both your models and your horse models on. However you have to change your eqclient.ini file to read AllLuclinPcModelsOff=TRUE
I bought the cheap 9k horse about a month ago and did the above. I move at Run3 speed with it. If I have SoW I run SoW speed. If I have a bard in my group I run at Selo speed. I can duck to interrupt casting. I can swim without losing my horse. I don't have its head filling a big part of my screen. I have no problems whatsoever with looting, or targetting, nor any acceleration and deceleration problems.
That said, I still get standing mana regen - it doesn't update my mana automatically as if I were sitting, but I just right-click my bridle to get the updated mana amount (I have now gotten into that habit and don't mind it at all). I can even "sit" to mem a spell in the middle of a pack of mobs and not get sitting aggro.
OMG I love my invisible horse.