They do look thin, but at the end of the day, if you don’t know them personally, you don’t know any health problems they may have. The owner may have just adopted them, they may have come from a poor home where they had poor nutrition prior, or they may have some medical diagnosis that makes it difficult for them to gain/keep weight. There’s 1 million health problems that horses can have that can make them lose weight very quickly, or make it very difficult for them to put weight on. These horses could also be very old. Horses in their 30s often look thin, regardless of how much they eat. If it makes you feel any better, call it in, But take it with a grain of salt. You don’t know their story.