Jarcast and I have had the same discussions about the Pasture Luxuries.
The first point is this one about improvement balance. Should there be Pasture Luxuries at all? (open question, but if you say no then obv you can just skip this part of the discussion)
On the particular choices:
Camels are a region variant of Horse, but they cannot be a Strategic for gameplay reasons so its either Bonus or Luxury. Of course Bonus is fairly boring and Luxury has monopolies, this motivates a lot of it.
Jarcast told me camel milk is a thing (citation needed). Having it as luxury also means you get lots of Camels together (looks cool), and it competes with the Incense desert start which adds variety.
Camel adds Food to Desert. Now a) is this desirable and b) if it is, should it be only some (Luxury) or all (Bonus).
Here I see it both ways.
Alpacas are a regional variant of Sheep, so why not a Bonus?
I don't know if I would like Alpacas everywhere, they are not as abundant as sheeps or goats worldwide (which nevertheless doesn't make them a luxury).
The argument here is that it exactly does make them a luxury. Ultra rarity of the wool. This is why we put the building bonus on Customs House (later export building, c.f. Cotton).
Again it also adds some Food to otherwise low-Food area (Hills region). Pros and cons there.