I have a Sheltie, a breed that herds by nature. They've been known to herd children in their family. That being said, my dog has tried to herd our horses since she was tiny. She's still tiny (at only 20 lbs) and she's much older now (just turned 11 years in December), but she still tries to herd them, nipping at their heels and sometimes jumping up to catch their tail. I've tried everything I can think of to break this habit, but once she gets that herding mindset on, there's nothing that can distract her.
My mare, Ginger, could care less most days. Sabrina, my Sheltie, is more or less a fly on Ginger's radar. She only really pays attention to Sabrina when I have grain, and she's at the other end of the pasture. Then all Sabrina has to do is run once behind her, barking, and Ginger will tear out across the field.
Duncan, who will be a year old this month, is a little different. He likes to play with Sabrina, and my poor dog just doesn't want to play back. Once she was sniffing at the ground when Duncan came up behind her to investigate. As soon as Sabrina realized she was being sniffed at by a horse she jumped and bolted. Duncan followed, bucking and running to catch up. Now every time he sees Sabrina in the pasture, and he's not distracted by grain, he runs up to her. I don't think she reciprocates his feelings, because she usually runs to the other side of the fence. hahaha.