If a magus wants to attract a beast by magically creating an appealing scent, would this be easiest to be an Aurum (add the smell property to the the air), Animal (create the odour part of the animal), or Imaginem (scent species) spell?
Either creo imaginem or go fiorect with rego animal and call the scent a cosmetic affectation
"Creating an appealing scent" is Creo Imaginem. Auram might affect how smells disperse through the air or make them smell duller or sharper (phenomena due to wind and humidity) but not actually create a smell. You could creo animal a scent gland from an animal and hope it attracts the creature through its scent, but as the base for a single sense illusion is 1 the CrIm way is easiest for anyone who doesn't have deficient form: Imaginem.
Couldn't you just use the existing spell Taste of the Spices and Herbs from the core book? Make a pitcher of water smell/taste as you wish and then splash it wherever? Or use a smell only variant with Moon Duration - it's still only level 5.
If you look at the science behind it it is a pheromone produced by the animal that attracts a animal of the opposite sex. I have yet heard of a animal that use small to attract a animal of the same sex. Also I think it is mainly females that use smell to attract a partner, I cant think of a male animal that do that. Males usually just use pheromones to mark there territory to let other males know not to come there.
Here to the tricky part. Would you say that the pheromone is a part of the animal. I would argue that it is not, the same way that excrement, stomach acid and saliva would not be considered "part" of the animal. It in all likeliness is a organic compound as it is produced by a animal.
So it is probably a Creo Corpus with requisite Animal or a Crea Animal with a Corpus requisite (I am not super up to date with 5e rules if they use requisite or not, I play 4th edition).
But I can see a bigger caveat with luring a animal this way. And that is in mythic Europe things that the people know exist exist. And no one up until very recently knew what pheromones is or how they work. So how a mage would figure it out I do not know. Most likely they would have to spend several years doing experiments to reach any sort of conclusion that eventually would lead them to super small particles that can´t bee seen that is dispersed in the air. Oh forgot, can´t be smelled by humans either.
A far more believable, and doable, way would be to use a possession spell. Possess the animal and walk it into a cage or where you want it. Or another way would be to invent a "speak to animal" spell and convince the animal to help (depends on what you need it for) or simply trick it. Probably very few animals would understand the concept of lying. It depends on what type of animal it is or why you need it.
Another way would also be to simply "create" a animal. Creo Animal.
creating milk is considered creo animal, so I would think pheromones would be, though they would just call it "musk" in the medieval paradigm.
I'm not sure if a pure imaginem spell (lets say it has example parameters T:Sun:Ind) would be enough to catch your beastie. You can make whatever the touched subject is smell as appealing as you like, but that doesnt cover much area, and you want to maximise your chances of having your creature detect the scent in the first place. Perhaps adding magnitudes to the spell would let the scent travel further (at your gm's discretion).
I think a CrAu(Im) spell would work best - create a gas of individual size (or greater, if you have magnitudes to spare) and the requisite make the gas smell of whatever you like. That way you can cover a ridiculously large area, what with the auram base individual size being enormous.
I would accept either animal or imaginem with an animal requisite (but only if the effect was intended to attract animals, not if it was meant to just create the scent of an animal). The animal effect would be creating the animal scent, where the imaginem would be creating the facsimile of the animal scent.
Auram is a valid form: "Auram spells usually affect air as phenomena (winds, odors) rather than as gases (a modern concept)", p125 core rule.
... followed on the same page by CrAu10: Stench of the Twenty Corpses.
The trick is going to be to create a smell that lead your target where you want, and not a spell covering a very large area - which won't lead accurately the beast anywhere.
Creating a potent smell through CrAu in a small jar, let it leak to attract your prey should work.