My little hippo gets these types of rashes too… ESPECIALLY in the summer.
We’ve found Antiseptic/Antifungal Shampoo helps tremendously! It’s a bit of a pain, but use it twice a week (leave on for 10 minutes before rinsing) until his skin visibly clears and once a week after for a few weeks then once every 2 weeks for maintenance. And a daily wipe down with Chlorhexidine Wipes or Spray on the problem areas daily keeps our girl much happier and less itchy.