A creature or object brought into being or transported to your location by a conjuration spell cannot appear inside another creature or object, nor can it appear floating in an empty space. It must arrive in an open location on a surface capable of supporting it.
Take the shaman and heaven spirit magic you can have the stars in your frog. Or at least an accurate map of the nightsky on its skin that moves to reflect where you are. Handy for navigating by the stars in a cloudy night.
I was expecting some "I'm a Diminutive Cavalier riding a frog that's actually Gargantuan and my Spirited Charge lance pounce is 432 avg damage at level 10" cheese, but for once I'm not disappointed by the thread not meeting my expectations.