Starglim said:
Casting Tongues on the hippogriff would work, and also allow it to speak.
Good point - I was only considering casting Tongues on the Druid, which wouldn't let him communicate with the Hippogriff. Your way works.
edit: If all Celestial creatures can speak Common then I've been playing Summon Monster very wrongly. I can't see from the SRD that they gain any specific language, including Celestial, which they did have in 3.0.
Abilities
Intelligence: A creature can speak all the languages mentioned in its description, plus one additional language per point of Intelligence bonus. Any creature with an Intelligence score of 3 or higher understands at least one language (Common, unless noted otherwise).
Celestial Creature
Abilities: Same as the base creature, but Intelligence is at least 3.
Neither the Celestial Creature nor the Hippogriff description 'notes otherwise', therefore a Celestial Hippogriff, with an Int of 3, understands Common.
Likewise, a Fox's Cunninged Hippogriff, with an Int of 6, understands Common.
-Hyp.
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