It is nothing, "gross," though it may sound like it. (Think shrimp or crayfish.) Mang dah is a rice beetle up to four inches long which makes its appearance in many Thai dishes and sauces. It is (especially the prised males) very aromatic (more like cheese than spice), and is eaten raw, roasted, and in many sauces. It is unlikely, however, you will find them in any Thai restaurants in this country.
The peppers in the picture and in your pantry are the dried version of priky nhu. Fresh, they come in a variety of colors, deep green, yellow, orange, and red, like the dry ones. They are frequently sliced into little rings and used in dipping sauces. I always have a bag of them in my refrigerator and another in my freezer (they keep well frozen up to 6 months or more). I frequently cut up four or five and sprinkle them on noodle dishes, or even a salad.
I use the dry ones on pizza. I used to have a bag of them where I worked, and when the company sometimes bought pizza for a meeting I would take them to put on my pizza. Despite my warnings there was always someone who insisted they liked hot peppers and would put them on their pizza. They seldom stayed for the whole meeting.