Recently a situation came up in one of my games where an animal was acting unusually intelligent so one of the player characters cast Detect Magic to identify whether or not the creature was of magical origin.
Is there a reason that would cause a familiar to be considered a Magical beast because it's more intelligent than a normal animal? Or perhaps was a creature that was summoned and that makes it different from normal animals of the same type perhaps?
Detect magic can only detect creatures — even highly magical creatures such as outsiders and elementals — if they are the result of an ongoing spell, such as summon monster. No creature type detects as magical in itself, not even undead. Specifically, the spell has listings for magic items and for spells, and for nothing else.
Familiars (emphasis added):
A familiar is an animal chosen by a spellcaster to aid him in his study of magic. It retains the [majority of the game stats] of the normal animal it once was, but is now a magical beast for the purpose of effects that depend on its type.
Generally, any animal that becomes too smart for the confines of the Animal type (2 Int or less) becomes a Magical Beast by virtue of specific wording of whatever effect is responsible for the Int increase; there is, as far as I know, no general rule to ensure this in all cases.
