One of the stranger monsters mentioned in the twelve tasks of Hercules, Geryon is described, in all the source material I've seen as having three bodies. I've always had a little trouble picturing such a being; the only material I've found with more detail is the book Geryon by Bernard Evslin describing Geryon as 'siamese triplets' (and he had a habit of taking great liberties with the source material, for examples see his other works Minotaur, Anteas and the Caledonian Boar just to name a few). Is there any existing works from ancient Greece that describe Geryon with more detail or show illustrations of him?
"Siamese Triplets" sounds about right, really. Apollodorus Library contains a textual brief description of his anatomy:
This island was inhabited by Geryon, son of Chrysaor by Callirrhoe, daughter of Ocean. He had the body of three men grown together and joined in one at the waist, but parted in three from the flanks and thighs.
Apollodorus Library, 2.5.10
Stesichorus's Geryoneis also gives him wings, as well as six hands and feet, as seen here.
And Aeschylus descibes his accoutrements:
Starting thence, when that he had crossed the ocean in a golden bowl, he drave the straight-horned kine from the uttermost parts of the earth, slew the evil herdsmen and their triple-bodied master, who wielded three spears in his (right) hands; in his left, extending three shields, and shaking his three crests, he advanced like unto Ares in his might.
Aeschylus's Hêrakleidai Fragment 37
Aside from that, there are some surviving visual depctions. Here are a few (all from about 550 - 500 BC):