Some mobs that are spawned on top of fire (spawned by commands or spawn eggs) will jump on top of fire and will never wander away from the fire. If you knock the mob back while they are on fire, they will pathfind themselves back to the fire and burn.

The following mobs are:
-Turtles
-Pandas
-Polar Bears
-Horses
-Donkeys
-Mules
-Zombie Horses (Sometimes they don't)
-Skeleton Horses (Sometimes they don't)
-Iron Golems
-Ravagers

To Reproduce
1. Use flint & steel or fire charge for fire
2. Spawn one of the following mobs on top of fire

Note: The iron golem is able to attack any hostile mob, but they will not chase them as they are standing on fire. Skeleton and zombie horses sometimes don't jump around the fire. Babies mobs aren't affected.