A Sprite possessing an adept can use the adept's powers with no limitations. I figure it is much the same with something like a hell hound.
But for something like a Merlin Hawk I could see it become more of a game balance issue.
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B A R S C I L W EDG ESS M Init IP
1 7 3 1 4 6 4 4 3 6 5 9 1
Movement: 30/70 (flight)
Skills: Astral Combat 3, Flight 5, Perception 4, Spellcasting 4,
Summoning 4, Unarmed Combat 3
Powers: Animal Control (Avians), Dual Natured, Natural
Weapon (Claws/Bite: DV 2P, AP 0)
Weaknesses: Fragile (1), Vulnerability (Willow-Wood Weapons,
Poisons)
Note: Merlin Hawks can cast any Detection or Illusion spell.
Of course the downside to using those spells is that the spirit is inside a body that can take vary little damage.
Also the hawk can simply be trained to cast the spells on command, and it would be less of a hassle to simply learn the spells. Plus because it did say they learn spells they probably take drain, and if that is the case then it probably is to much like a possessed mage an spells.
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Free Spirit possessing a Cerberus Hound
This probably is the best bet. I have been looking into it too. The other obvious choices are Fenrir Wolf (means fen dweller, they are easier to get a hold of but are totally illegal) or Hellhound(because they can get a license and so potentially not be shot on sight). Cerberus Hound gets so many useful abilities. But I just looked at the cost. It is (80,000¥x1.25x2) or 200,000 just to get it with instinctive training. It is only 80,000 with out the training that you can do in game if you have the skills.
The other thing to consider is that wile you are possessing a vessel you use YOUR magic attribute but the lowest of the two mental attributes, and because of limited starting points a free spirit is likely to be the less mentally gifted of the two. By a wide margin too.