We really need more info before we can help you out.. Is she rugged or un-rugged, is she fed any special food, is she grazed with other horses, does she eat hay, what are her teeth like etc.
With arthritis, I would say she is probably best outside too.. If she is getting cold, make sure she has a nice warm rug on and plenty of hay to keep her warm, if the arthritis is mainly in her legs then maybe you could put those fluffy paddock boots on to keep her legs warm as well.
What is her weight like? Is she overweight/underweight/right on target?
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms