Had my horse a week all was ok when I bought her home she was in a field on her own and 2 days later I found scabs on her back appeared over night not sure how to add pictures
Rain scald usually form small(5mm across) scabs at the base of the hair, and when you pick them off, it he hair comes out in a tuft with the scab at the base, and it may or may not be a bit 'oozy' under the scab. Sometimes big patches form, leaving the horse bald and sore.
It is caused by a bacteria which likes damp conditions.
Has you horse been sweating, even a little, under his rug? Have you been washing him?
If it is rain scald, you will need to use a medicated solution to bathe the areas, and really get under the scabs. I have found tea tree oil good in mild cases, but it might be best to check with the vet.