Frukathka said:
Comprehend Languages, page 212, 3.5 PHB
"You can understand the spoken words of creatures..."
Do you allow CL to work on a dog or a bear, for example? Or do you require Speak With Animals? There are no 'spoken words' to understand.
And besides, CL only allow
you to understand, not the subject creature to understand you.
"Tongues does not enable the subject to speak with creatures who don’t speak."
So Tongues is no good either.
Your best bet is perhaps Fox's Cunning followed by Rary's Telepathic Bond.
(Note that the Fox's Cunning by itself will allow the Hippogriff to understand
you... its Intelligence will now be 6. "Any creature with an Intelligence score of 3 or higher understands at least one language (Common, unless noted otherwise)." ... and the hippogriff entry does not 'not otherwise', so as a creature with an Int score of 3 or higher, your Cunning Hippogriff understands Common.)
-Hyp.