I would call the local humane society, but that is just me. That mare looks VERY malnourished and underweight. I would not ride the first horse, because its complete lack of muscle worries me.
As far as the 'vibrating' goes, some places just have more bugs and flys that bother the horses. But if that is the case, then they should put more fly spray on the horses, or fly masks or SOMETHING. The horse looks like its being eaten alive by the bugs. =(((
He looks like he's probably older. He could use a good fifty pounds on him, but he's not horribly skinny. Just walking around on him shouldn't hurt. There's a 30 year old mare at my barn that's the hardest keeper I've ever seen. She's a little underweight, but the only person that usually rides her is a 7 year old. She's very well cared for, though.
If the other horses don't look like that, I don't think it's a neglect issue. I would ask around and see what the deal is with that horse just to make sure. If it sounds shady, I'd get the heck out of there and call the humane society.