My vote is brook-brown X. Dont know how as the Brown is salmonoid and the bull is char family, kinda two different families but I am not a biologist. I did find this though...Google
Hybrid populations of char occur in some North American habitats, such as the splake (brook trout x lake trout), the brook trout x bull trout hybrid, and the tiger trout, a cross between brook trout and nonnative brown trout.
I also know I used to catch tons of brookies in that watershed along with the bulls and rainbows. Dungarven has lots of brookies.
A rare fish to be sure....especially if occuring naturally...