Horses who like each other spend a lot of time grazing side-by-side. It is also common to groom each other and when doing this they use their front teeth to scratch each other.
When they are mutually grooming each other, they are scratching each other and nibbling along each other's withers, crest, back and croup.
Horses do this to bond with each other and they live by the philosophy 'you scratch my back and I'll scratch your.'
Here you see Toronto and Oscar scratching each other: