Need help. Unhealthy horse?

I'm not a pro when it comes to horses but this troubles me. I went to a barn today because it's my birthday and I wanted to see em horses and perhaps ride em. I want to go riding but I'm not sure if they are healthy enough to be ridden.

Main horse which lotsa people have ridden and will be the one that I ride if I do get to ride him. His back worries me.

He looks fine, just older and a bit swayed. As long as they use the right saddle pads and saddles, he should be fine.

The damage has been done many years ago. It won't hurt him more to ride him.

My horse to me is thin so I feel bad about riding for more then 10 min. His back well maybe if he had more weight he wouldnt look so bad.

The video is short, and veiw blocked, but it looks like he is awfully thin, and feet may be fairly overgrown...He might be older, but he might just not be very well cared for; did all the horses look like this?

No, not all the horses were like this. This is either the only rideable horse (the rest are unbroken colts) or he could be one of the three rideable horses in the whole place.

He is kept with 2 other horses and they look different from the other horses kept in different stables. The other horses (about 40) look well fed though and no ribs can be seen.

I'll upload another video of a second horse (one of those staying in the same area as the horse in the first video). Just need to get Youtube working.

Video of the second horse.

The horse in the second video is pregnant. :O

She looks thin to me if she is prego. Im sorry but I feel like they are not very well taken care of that's just my op.

The horses are thin. The first one looks older and is probably ok to be ridden, the second one, if it's pregnant it's way too thin.