Wrahn, knowing that shapechanging spells are a pain in the ass I´ve prepared beforehand the stats and changes of some of the most useful forms to take; those are cretures in the monster manual that either Tares automatically knows or has to roll very low to know (the highest, a 3+ for the cornugon) There are more powerful outsiders, but they need a better roll.
The question is, since horned devils use weapons and are described as "humanoid" -though they have a tail and wings-, I think they can wear equipment (that is, equipment doesn´t meld into that form when shapechanging), but I´m not really sure.