Hi and welcome to the forum
The fish looks like it has pushed part of its intestine out of its butt. This happens sometimes and the intestine normally goes back inside the fish after a few hours. If it doesn't go back in after 24 hours you can carefully and gently catch the fish in a net and gently rub your finger along the belly a couple of times. Do this very gently otherwise you can harm the fish. Keep the fish under water when you do this.
Feeding the fish too much dry food can encourage this, especially when fish are inbred like guppies. Feed the fish a varied diet consisting of dry, fresh, frozen (but defrosted) and live foods to help avoid these problems. Make sure there is some plant matter in the diet or grow algae on the glass or ornaments so the fish can graze on that.
The fish looks pretty skinny too, especially around the face. It probably has gill flukes and intestinal worms. You can try treating the tank with Praziquantel (available from pet shops) to kill any gill flukes and tapeworms in the fish. And look for some Levamisole to treat thread/ round worms in fish.
*NB* Do not use the medications at the same time.
Treat all the fish in all tanks at the same time. You treat them once a week for 3 weeks so you kill the adults and any baby worms that hatch from eggs in the fish. Do a 75% water change and gravel clean the substrate 24-48 hours after treatment. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.