Glowfly

Here it is.
I highly recommend though that you buy a Bugnet from the pirates first before you try to catch them.
There is a 50% chance to catch one but you can only try once an hour. Because it's just an very exhausting job.
Oh and don't be blinded. ;p

Now for the real thing. I don't know when the spirit world opens but there is a chance for you to catch Spirit Fireflies there.

What do they do? o.O Do they have different forms?

Not really. The Glowflies only come in that size, color and shape. They're unbreedable adults.
The Spirit World Fireflies come in 2 different versions. Green and Blue but just because I couldn't decide wether they are green or blue. xD maybe I will make a teal/cyan version also but for now that's all.

Maybe you could make more, like a pink version