Ooo, nice.
Variation on this idea: Would they still be . . . able to be bought in cages?
Bought, yes, as the horror of the beauteous creature being caged would be counterbalanced by the glory of setting it free. What
wouldn't be tolerable would be allowing the merchants to leaved with the creature
still caged & in their possession--you'd either have to purchase it, or seize it outright, and in either case setting it free to live on your site. Which might not be feasible: It might be a grazer that eats up all of your scare pasture-land so that your
other animals must starve, or perhaps you can't guarantee its safety from goblins or Evil weather or whatnot. Unfortunately, once a Tame animal is yours, it's trapped on your site for the rest of its life--AFAIK, there's no way for it to leave the map. (Unless you manage to knock it unconscious over a Cage Trap & ship it back to the Mountainhome, I guess.)
Also to be considered: Possibly being able to shear/butcher a sacred animal that died of natural causes/attack by predators. (You'd still probably only be able to use the meat & products for special ceremonial purposes, though.) Conversely, the question of being able to shear a
dead creature, but not a live one.
Another possibility: Having the creature be considered sacred not by an entire civilization, but only by members of the civ that also venerate a specific deity.
Supplemental Suggestion: Give Animal Trainers a way to "
un-domesticate" Tame animals, causing them to revert to a semi-wild state. Besides the above scenario of essentially shooing sacred animals off the map, it would be a useful way of making ordinarily docile animals more territorial, so that they'll be more aggressive, and in general put up more of a fight, when other creatures wander onto the map. If I've already got
more than enough llamas to supply my food & cloth industries, I
should be able to train the extras to at least scare off the occasional flock of keas.