You've got an extremely rare find there Jethro69. That fang is a genetic throwback. Before whitetailed deer were whitetailed deer--before they had evolved to their present physical shape, which occurred around 10,000 years ago at the end of the last ice age--the precursor species to whitetailed deer had fangs instead of antlers. Whitetailed deer still carry these genes for fangs but those genes are normally unexpressed (the genes are not active). However, every so often and for reasons unknown these genes become activated, causing the deer to grow his/her ancestral fangs.
The same thing happens with manes. The precursor species to whitetailed deer had short manes that ran up the back of their neck. These genes are normally inactive, but on rare occassion they "turn on" causing the deer to grow a short section of mane.