Can Polymorph be cast on a Familiar?
What a creature can be polymorphed into depends on the creature's level or CR, of which a familiar has neither. Unless you use the CR of the form the familiar has taken.
Or, as a spirit that takes the form of a creature, is a familiar a shapechanger and therefore not a valid target for the spell?
If you can polymorph a familiar, is it still unable to attack?
Find familiar states:
Appearing in an unoccupied space within range, the familiar has the statistics of the chosen form, though it is a celestial, fey, or fiend (your choice) instead of a beast.
The CR is one of the statistics of the form, so the familiar has the same CR as the chosen form (which is 0 for most forms). Moreover, being a shapechanger is also a statistics, so the familiar is a shapechanger only if the chosen form is a shapechanger (as is the case for some of the Pact of the Chain additional familiar options).
Polymorph states:
The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast.
Find familiar describes several effects, including the fact that the familiar can't attack, it disappears instead of dying, it always obeys, etc. but none of these effects are implied to be statistics. Hence, when you polymorph the familiar, all of these effects are not replaced like statistics and the familiar remains incapable of attacking.
The description of the polymorph spell simply says it targets a creature:
This spell transforms a creature that you can see within range into a new form. [...] The spell has no effect on a shapechanger or a creature with 0 hit points.
[...] The new form can be any beast whose challenge rating is equal to or less than the target's (or the target's level, if it doesn't have a challenge rating).
As noted in the official ruling on whether a familiar counts as an ally for Sneak Attack, a familiar is a creature.
The description of the find familiar spell says:
You gain the service of a familiar, a spirit that takes an animal form you choose: bat, cat, crab, frog (toad), hawk, lizard, octopus, owl, poisonous snake, fish (quipper), rat, raven, sea horse, spider, or weasel. Appearing in an unoccupied space within range, the familiar has the statistics of the chosen form, [...]
The CR of a familiar from the spell is, in fact, that of the chosen form.
A familiar is not considered a shapechanger just because it's a spirit that takes an animal form. A creature that is considered a shapechanger will have that monster tag appear in parentheses following its type (as seen in the doppelganger's statblock).
Note: A familiar still cannot attack even in its new form, as it is still a familiar. Per the spell description:
[...] A familiar can't attack, but it can take other actions as normal.
Note 2: If you dismiss your familiar or it is reduced to 0 hp, and then you resummon it, the familiar is brought back in one of its original forms, and not in the form you chose with the polymorph spell.
Yes, but you can't polymorph into something of a higher CR than the transformed creature. Most familiars are only CR 0 I think, so there's not much use.
