Ninja-to said:
I don't understand how a Celestial animal knows common when no intelligent creatures on that plane would ever speak it. They'd choose to speak Celestial first over Common, even if they can speak Common.
Well, because:
All the following from the SRD:
Angels: Angels speak Celestial, Infernal, and Draconic, though they can speak with almost any creature because of their tongues ability.
Archons: Archons speak Celestial, Infernal, and Draconic, but can speak with almost any creature because of their tongues ability.
Bralanis: Bralanis speak Celestial, Infernal, and Draconic, but can communicate with almost any creature, thanks to their tongues
Demons: Except where otherwise noted, demons speak Abyssal, Celestial, and Draconic.
Devils: Except when otherwise noted, devils speak Infernal, Celestial, and Draconic.
Ghaeles: Ghaeles speak Celestial, Infernal, and Draconic, but can communicate with almost any creature, thanks to their tongues ability.
So, given the argument that "Celestial is spoken in the upper planes, and therefore magical beasts from those planes should understand Celestial, not Common," an equally strong argument could be made that every major creature type on all the outer planes speaks Draconic, and therefore magical beasts from those planes should speak Draconic.
Furthermore, there's an eminently sensible reason for Celestial and Fiendish creatures to understand Common: it's what the majority of things that summon them will speak, and the outsiders from their home plane can talk to them anyway via their
tongues abilities.