Kerberos, the Multi-headed Hound of Hades
The final and most trying labor, which took the hero straight to the heart of the underworld - a place to which no mortal was to descend, yet return. 
Indeed, its lack of practical value - a mere challenge to bring the fierce three- (or more) headed dog guarding the underworldly realm of Hades - reflect king Eurystheus' confidence of Herakles' failure. Needless to say, this didn't end up being the case, and our hero, guided by Athena and Hermes, wasted no time to perform some further good deeds in the process (e.g. free Theseus, feed the dead souls). As usual, it is not easy to find depictions of this showdown - one example is a colonial bronze from Hadrianopolis (Thrace) under Gordian, typically known from a single characteristic reverse die - with a confident looking Herakles nonchalantly approaching the beast - somewhat in contrast to the fierce grappling scenes common for the first labor against the Nemean lion (perhaps experience and confirence built uo over the course of a dozen labors). On the other hand, Kerberos himself - a mythological character with significance beyond just this story - does appear somewhat more frequently on coinage, with a varying number of heads.
Search for coins depicting Kerberos.