I'd never even considered jealousy (our lot are the 'haha you're about to get WORKED' type that laugh over the fence at whoever's in the ring) until one day we went out to catch the boys for a hack. My friend couldn't catch her boy, and since I normally take a couple of treats into the field with me to help catch Geoff, the boys all know and love me. So she caught Geoff while I caught her horse - and Geoff kept trying to spin round to see where I was, and kept stopping on the way to the gate.
I don't think it was outright jealousy, more like 'this isn't the way we normally do things'! Or else the whole 'is he getting my treats' thing. But my last horse was so sociable with everyone and generally attention seeking with everyone that I wasn't expecting that reaction from Geoff... funny :) Then again, he does neigh to me now from the stable, so I'm obviously his pal these days :)