Over at One Coin at a Time, Brett Telford writes:
The cruelty and executions during his reign of terror were so odius that he [Domitian] earned the nickname “the Beast” amongst Romans, Greeks, Christians and Jews, according to Ethelbert Stauffer in Coniectanea Neotestamentica XI in honorem Antonii Fridrichsen sexagenarii. Ethelbert Stauffer was a German Protestant theologian who held that gematria, the numerology of the Hebrew language and alphabet, could be used to explain the Biblical number 666. Stauffer computed this “Number of the Beast” using the short form of Domitian’s names and titles: Imperator Caesar Domitianus Augustus Germanicus… which in Greek is: Autokrator Kaisar Dometianos Sebastos Germanikos. The latter abbreviates to A KAI ΔOMET ΣEB ΓE and the gematrical formula reads:
A. K A I. Δ O M E T. Σ E B. Γ E. 1+ 20+1+10+4+70+40+5+300+200+5+2+ 3+5 = 666
Telford notes that gematria formulas have also been used to link 666 to Nero.