Dengus trolls were a special useless celebration monster for the departure of the previous product manager.
They randomly fuse + to the next for or not... you can upgrade them to Dengus Troll lvl 4 (UR)
Apart from that they are pretty useless.
Heard that sometimes they fuse+ in something useful but never seen it myself so as far as 'm concerned, these are rumors.
I use them as fuse food.