At this point, I haven't been able to find the specific text that precludes removing the effects of the gift. In fact, I stand by the Nightmare Horse example as explicitly stating the opposite. If somebody can find the text though, I'll humbl accept the correction.
This doesn't make sense to me. If anything, the "issues" a Nightmare horse has to deal with should make it harder to be ridden by anyone, don't you think? This is a beast noted for its ferociousness, after all.
It does suggest an amusing scenario, though: "Quick, the dog is too busy being rabid to bite us! Give him a bath!"
Well, I continue basing this off the two spells in the text that deal with the issue. They seem to imply that Gift sensitivity is a quality, not a state (like a mood or some such), and as such a beast can be created with or without it (Wizard's Mount), or changed in such a way that they have it or not (Steed of Vengeance). Rego can manipulate people and beasts in such a way that they act as if they didn't have the quality of Gift sensitivity, but they still have it. Also, note that Rego is not given as a requisite in either of the cited spells, which it would be if it was neccessary for the extra effect. For instance, if I were to create a spell similar to Wizard's Mount, let's call it
[i] [The Clown's Mount
Muto Animal 35
Creates a horse that is normal in every way except that IT IS ALWAYS HAPPY! SO HAPPY HAPPY HAPPY! [/i]
we'd need a Rego requisite, right? And it's lack would be a glaring omission, because being happy" isn't something that Muto can manipulate. But a quality, like, say, wanting to eat grass instead of meat could be manipulated with Muto, although the change would not be natural.
Anyway, that's my reasoning. I appreciate the feedback so far. Feel free to poke holes in my thought process.