Do you have any exercises to make my mare's neck thicker (she's a very hard keeper, but at a good weight now but she has always had a really thin)? Thanks!

I would suggest flexing and lots of bending (into corners, circles etc)!
You need to work on her topline. Do exercises that will require her to engage her hind end such as lateral work, caveletti work, and pole work. Hill work will also be helpful, but what really matters is good flat work. At 18, as long as it doesn't strain her, you can also play around with small gymnastics.
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