The Zug bug is a fly that works really well, however it doesn't seem to resemble any particular bug specifically. Not sure really what it is supposed to mimmic. Some say caddis (cased or pupa), and while that could be, it really isn't a good representation. However, that means nothing in fishing. Something doesn't have to look identical to a forage for a fish to eat it. And trout love this fly. I find especially in the winter time for some reason. Although I have had them take this fly year round.
The fly is also relatively easy to tie compared to some other flashy patterns like copper johns or prince nymphs (both of which also don't mimic anything specific, but trout love them!).
As always I am listing all the materials used on this fly below.
Hook: Firehole Sticks #609 in size 14
Thread: Veevus 6/0 in Black
Tail: Peacock Swords
Ribbing: Silver Tinsel
Body: Peacock Hurl
Hackle: Hen neck or saddle
Wing Case: Natural Mallard Flank
Head Cement: Original Hard as Hull