Don't worry. You're just as sane as I am
-Luna Lovegood

It's alright" said a dreamy voice from beside Harry as Ron vanished into the coach's dark interior. "You're not going mad or anything. I can see them too."
"Can you?" said Harry desperately, turning to Luna. He could see the bat-winged horses reflected in her wide, silvery eyes.
"Oh yes," said Luna, "I've been able to see them since my first year here. They've always pulled the carriages. Don't worry. You're just as sane as I am."
Smiling faintly, she climbed into the musty interior of the carriage after Ron. Not altogether reassured, Harry followed her.

Hello, Harry!” she said.
“Er — my name’s Barny,” said Harry, flummoxed.
“Oh, have you changed that too?” she asked brightly.
“How did you know — ?”
“Oh, just your expression,” she said.
Like her father, Luna was wearing bright yellow robes, which she had accessorized with a large sunflower in her hair. Once you got over the brightness of it all, the general effect was quite pleasant. At least there were no radishes dangling from her ears.

My mum always said things we lose have a way of coming back to us in the end. If not always in the ways we expect".
-Luna lovegood( Harry potter and the order of the phenoix)

I suspect Nargles are behind it.

Harry then did something that was both very brave and very stupid