Our Aero came to us with no idea that she could eat apples or carrots. She would gladly eat the packaged horse treats all day long. We would even try putting them in her grain - only to find every single bite of fruit/veggie, sucked clean of every last bit of grain, in her tub at the end of her meal.
Holly, on the other hand, hasn't met anything she hasn't tried to eat -- she LOVES fruits and veggies. It was probably about three or four months after Holly came to live with us that one day Aero just reached over and took a carrot and ate it without blinking an eye. She's the same with apple now, too. I think she was just jealous that Holly was having something she wasn't - not sure she likes them even now, but she eats them like mad.
Honestly, though, it's not like they "need" anything anyway.