I’ve seen rain rot a lot and never anything that a vet needed to be called for. It clears up on it’s own once the animal is kept in a dry environment. Rain rot is caused by bacteria called dermatophilus congolensis. It’s normally seen in animals in wet rainy weather who have no shelter and end up remaining wet. If you live in an area that gets a lot of rain a water proof sheet can help prevent rain rot. Not all wet horses get rain rot. The bacteria has to be present so you may never see it even if all other risk factors are present.
Once the animal is removed from the wet conditions, a good bath with antibacterial shampoo works well. Do not share brushes as that will transport the bacteria, but will not infect another horse unless it is also left out in the rain. The hair will grow out and you will be able to curry away the rain rot. Unless nee hair has started to grow back in don’t just rip it all off….It’s not going to make it worse really but looks kinda painful so I wait til the new growth comes in before I start using a curry comb on the affected area.