I was thinking about it for the griffin rider paragon path.Not currently, no. You can get it as part of a hybrid ranger (which isn't approved, and likely not to be [playtesting material isn't ready for prime time]) but you don't get the ritual if I recall and the beast takes penalties to attack. Unlike the other forms of multiclass, the beast doesn't really grant you much unless you do power-swaps.
Is it just me or should the warhorse have about 150lbs more normal carrying capacity? As is, a standard knight (200-250lbs) in shining armor (50lbs) puts it at heavy load before it even gets its own armor. What ever happened to the heavy warhorse? The difference is obvious in the movie "the 13th warrior" between a riding horse and a heavy warhorse. The warhorse is almost twice the size overall. For that matter the whole weight chart is off in the AV. They have a triceritops at 325lbs for normal load? What a joke.