Because of the reflection of his eyes when the picture took. Cats have a luminescence or reflective matter in their eyes. When light shines at a certain angle in the dark, their eyes appear to glow. Monster kitty is just a term I use when I see this as it reminds me of monsters, like Frankenstein.

I know of a lot of cat owners who dub their kitties the same way when they see the phenomenon. Cot really isn't a monster, except every now and again, lol!

Thanks for asking and have a wonderful day!