A species of interdimentional predators exist in a separate reality from ours. They pass into different realities through portals which occasionally open up between worlds. Once there, they feed on the populace for a period of time, before returning to their reality to hibernate. The feeding cycle revolves around feeding off the emotions of young children, and takes place in two cycles.
The first centers around feelings associated with hapoiness. The creature takes the form of an individual meant to inspire love and joy among children, such as a beautiful female caretaker that travels on a flying umbrella. The being would do anything it took to inspire joy in these kids, such as sing along musical numbers, dancing, and making them feel good about themselves, all the while feeding on their emotions. When it has filled itself, it leaves to hibernate for 25 years.
It's next cycle involves feeding on emotions associated with terror. The creature takes a more sinister form, such as a scary looking clown. It works to inspire feelings of fear and horror in it's targets, such as showing them their worst nightmares or painful memories. This can take place over a certain period, until the victim has experienced a suitable amount of fear. Then the creature devours them alive. It then returns to stasis after its cycle finishes, until it's next cycle begins again.
These two feeding cycles are completely different and involve feeding of opposite emotions. These creatures must switch between these emotions after they come out of hibernation every 25 years. Why would a species evolve in this way?