Hypersmurf said:
...even a Wizard sharing a Barkskin from a Ring of Spell Storing or a Limited Wish with his familiar.
well, OK, but I would probably nix the Ring of Spell Storing
Hypersmurf said:
"You acquire the physical qualities of the new form while retaining your own mind. Physical qualities include natural size..."
The natural size of a Centaur is Large, not Tiny or Small.
"... and any gross physical qualities (presence or absence of wings...)"
You specifically inherit the centaur's 'absence of wings'.
"You can freely designate the new form’s minor physical qualities (such as hair color, hair texture, and skin color) within the normal ranges for a creature of that kind. The new form’s significant physical qualities (such as height, weight, and gender) are also under your control, but they must fall within the norms for the new form’s kind."
'Feathers' are not within the normal range of the hair texture of a centaur. Tiny or Small are not within the norms for a centaur, just as Mini-V is not within the norms for a human.
The feathers bit referred to the familiar being an owl. I figured that if the owl couldn't become a centaur, it would pick up some of the characteristics thereof, if only to retain its resemblance to its master. I don't have the PHB here, but I recall text saying that the master and familiar are essentially the same creature.
...
Despite this being the Rules forum, let's take a minute to set the game down, walk away, and thoughtfully consider the term "magical beast." Would a hyperintelligent owl temporarily adopting some of the traits of a centaur generally fall within the bounds of what you would consider to be a magical beast? Particularly as an artifact of another creature's self-altering magical effect?
...
Now that we're back in the game, let us again consider the term magical beast, in clumsy faux-Platonic dialogue:
Q: What makes it magical?
A: That it conforms so closely to all aspects of its masters behavior, and acts as a feedback mechanism for Master's power.
Q: What happens when its master outwardly transforms into an orc?
A: [what happens when master casts Alter Self]
Q: What happens when its master completely transforms into an orc?
A: [what happens when master casts Polymorph]
please edit at will. Describe the effects of the last two answers.