I used to have a ranch bred QH who was totally unfamiliar with treats. Even after he figured out what carrots and apples were (which were OK in his opinion, but nothing to do cartwheels over) he regarded most treats with deepest suspicion. Granola bars were acceptable, probably because they resemble sweet feed. He also liked dried sweetened cherries for some reason.
My horse refused all treats from my hand for months after I accidentally gave him a fig with bee inside it. Boy, did I get a major stink eye for that one. I felt so bad but he wouldn't let me make it up to him.
Currently he will not eat a carrot. He likes apples, and figs (again) and cookies but no carrots.
"My biggest fear is that when I die my husband is going to try to sell all my horses and tack for what I told him they cost."
Recipient mare and new mare from the race track both seem confused about treats. Neither will take sugar, mints or apples. Recipient mare also doesn't know what carrots are - poor girl! Stopped trying to put them in her feed because I got tired of cleaning them out and tossing them later.
However both will happily gobble up Mrs. Pastures Horse Cookies. I had to start them in the recipient mare's feed bucket but now she'll take them from hand.
My warmblood mare started out picky. It took her about a year to figure out sugar and apples but she LOVES mints. Little Debbie Oatmeal Cream Pies are another barn favorite, as are Cinnamon Pop Tarts.
One of my Clydesdales refused treats when he first arrived. The trainer even tried putting carrots in his mouth and moving his jaw so he would see it tasted good and change his mind. He was always eager to please but would not accept a reward. But he had been in pasture his whole life and not handled much at all, and he was 5 when he came. Then we went to a show and were stalled next to some nice folks with minis. One owner came over with cookies and asked if our horses could have them. I told her "surem but Tuffy won't take one." Then he made a liar out of me! Took the cookie and has been asking for treats ever since.
When jumbo shrimp came he did not know what treats were.
We started with carrot tops (i.e. the green stuff that grows out of the fancy carrots) and he soon figured out that humans bring yummy treats. Only took a week and now he eats first & thinks later. LOL