Ladon was the serpent-like dragon that twined and twisted around the tree in the Garden of the Hesperides and guarded the golden apples. Hercules killed him with a bow and arrow by Heracles. The following day, Jason and the Argonauts passed by on their chthonic return journey from Colchis, hearing the lament of "shining" Aegle, one of the four Hesperides, and viewing the still-twitching Ladon. In an alternate version of the myth, Ladon is never slain, and Heracles instead gets the Titan god Atlas to retrieve the apples. At the same time, Heracles takes Atlas’ place, holding up the sky.
Ladon was given several parentages, each of which placed him at an archaic level in Greek myth: the offspring of "Ceto, joined in heated passion with Phorcys" or of Typhon, who was himself serpent-like from the waist down, and Echidna. "The Dragon which guarded the golden apples was the brother of the Nemean lion" asserted Ptolemy Hephaestion.
The dragon (Ladon) image coiled around the tree, originally adopted by the Hellenes from Near Eastern and Minoan sources, is familiar from surviving Greek vase-painting. In the 2nd century CE, Pausanias saw among the treasuries at Olympia an archaic cult image in cedar-wood of Heracles and the apple-tree of the Hesperides with the dragon coiled around it.
Diodorus Siculus gives an euhemerist interpretation of Ladon, as a human shepherd guarding a flock of golden-fleeced sheep, adding, "But with regards to such matters it will be every man’s privilege to form such opinions as accord with his own belief." 
Ladon is the constellation Draco according to Hyginus' Astronomy. Ladon is the Greek version of the West Semitic serpent Lotan, or the Hurrian serpent Illuyanka. He might be given multiple heads, a hundred in Aristophanes' The Frogs (a passing remark in line 475), which might speak with different voices.
Classic Literature Sources
Dragon of the Hesperides
Chronological listing of classical literature sources for the snake, serpent, guardian or dragon of the Hesperides:
- Hesiod, Theogony 333 ff (trans. Evelyn-White) (Greek epic poetry C8th to C7th BC)
- Euripides, Heracles Mad 394 ff (trans. Coleridge) (Greek tragedy C5th BC)
- Sophocles, Trachiniae 1090 ff (trans. Jebb) (Greek tragedy C5th BC)
- Scholiast on Sophocles, Trachiniae 1098 (Sophocles The Plays and Fragments Part V The Trachiniae trans. Jebb 1892 p. 159)
- Apollonius Rhodius, Argonautica 4. 1390 - 1451 (trans. Coleridge) (Greek epic poetry C3rd BC)
- Scholiast on Apollonius Rhodius, Argonautica 4. 1396 (The Argonautica of Apollonius Rhodius trans. Coleridge 1889 p. 195)
- Aratus, Phaenomena 63 ff (trans. Mair) (Greek poetry C3rd BC)
- Scholiast on Aratus, Phaenomena 66 (Callimachus and Lycophron Aratus trans. Mair 1921 p. 386)
- Diodorus Siculus, Library of History 4. 26. 2 ff (trans. Oldfather) (Greek history C1st BC)
- Lucretius, De Rerum Natura 5. Proem 1-54 (trans. Leonard) (Roman philosophy C1st BC)
- Cicero, De Natura Deoeum 2. 106 (Mayor and Swainson) (Roman philosophy C1st BC)
- Scholiast on Cicero, De Natura Deoeum 2. 106 (Ciceronis De Natura Deoeum Mayor and Swainson 1883 Vol 2 p. 223)
- Cicero, De Natura Deorum 42. 108
- Scholiast on Cicero, De Natura Deorum 42. 108 (Ciceronis De Natura Deoeum trans. Mayor Swainson 1883 Vol 2 p. 225)
- Propertius, Elegies 2. 24a. 23 ff (trans. Butler) (Latin poetry C1st BC)
- Virgil, Aeneid 4. 480 (trans. Fairclough) (Roman epic poetry C1st BC)
- Scoliast on Virgil, Aeneid 4. 484 (The Works of Virgil trans. Hamilton Bryce 1894 p. 251)
- Ovid, Metamorphoses 9. 190 (trans. Miller) (Roman epic poetry C1st BC to C1st AD)
- Pseudo-Eratosthenes, Eratosthsmis Catasterismi Cap. 3 Draco (Eratosthsmis Catasterismi trans. Schaubach Heyne 1795 p. 18) (Greek mythography C1st AD)
- Pliny, Natural History 5. 3 (trans. Rackham) (Roman encyclopedia C1st AD)
- Lucan, Pharsalia 9. 358 ff (trans. Riley) (Roman poet C1st AD)
- Scholiast on Lucan, Pharsalia 9. 358 (The Pharsalia of Lucan trans. Riley 1853 p. 358)
- Seneca, Hercules Furens 239-240 (trans. Miller) (Roman tragedy C1st AD)
- Seneca, Hercules Furens 527 ff
- Seneca, Hercules Oetaeus 18 ff (trans. Miller)
- Seneca, Phoenissae 316 (trans. Miller)
- Seneca, Agamemnon 852 ff (trans. Miller)
- Silius, Punica 6. 183 ff (trans. Duff) (Roman epic poetry C1st AD)
- Ptolemy Hephaestion, New History Book 5 (summary from Photius, Myriobiblon 190) (trans. Pearse) (Greek mythography C1st to C2nd AD)
- Pseudo-Apollodorus, The Library 2. 5. 11 ff (trans. Frazer) (Greek mythography C2nd AD)
- Pausanias, Description of Greece 6. 19. 8 ff (trans. Frazer) (Greek travelogue C2nd AD)
- Pseudo-Hyginus, Preface (trans. Grant) (Roman mythography C2nd AD)
- Pseudo-Hyginus, Fabulae 30
- Pseudo-Hyginus, Fabulae 151
- Pseudo-Hyginus, Astronomica 2. 3 (trans. Grant)
- Lucian, The Dance 56 ff (trans. Harmon) (Assyrian satire C2nd AD)
- Philostratus the Elder, Imagines 2. 17. 34a ff (trans. Fairbanks) (Greek rhetoric C3rd AD)
- Philostratus the Elder, Imagines 2. 21. 5 ff
- Quintus Smyrnaeus, Fall of Troy 6. 256 ff (trans. Way) (Greek epic poetry C4th AD)
- Nonnos, Dionysiaca 33. 276 ff (trans. Rouse) (Greek epic poetry C5th AD)
- Scholiast on Nonnos, Dionysiaca 33. 276 (Nonnos Dionysiaca trans. Rouse 1942 Vol II p. 269)
- Scholiast on Caesaris Germanici Aratea 41B (Martianus Capella ed. Eyssenhardt 1866 pp. 382 sq.) (Roman prose C5th AD)
- Boethius, The Consolation of Philosophy 4. 7. 17 ff (trans. Rand & Stewart) (Roman philosophy C6th AD)
- Servius, Servius In Vergilii Carmina Commentarii 4. 246 ff (trans. Thilo) (Greek commentary C4th AD to 11th AD)
- Servius, Servius In Vergilii Carmina Commentarii 4. 484 ff
- First Vatican Mythographer, Scriptores rerum mythicarum 38 Hesperides (ed. Bode) (Greek and Roman mythography C9th AD to C11th AD)
- Second Vatican Mythographer, Scriptores rerum mythicarum 161 Aurea poma (ed. Bode) (Greek and Roman mythography C11th AD)
- Tzetzes, Chiliades or Book of Histories 2. 355 ff (trans. Untila et. al.) (Greco-Byzantine history C12th AD)
- Lernaean Hydra, a similar monster who was also slain by Heracles.
- Argonautica, Book IV.
- Hesiod. Theogony, 333.
- Bibliotheke 2.113; Hyginus, Preface to Fabulae.
- Recorded in his New History V, lost but epitomized in Photius, Myriobiblion 190.
- Pausanias. Description of Greece, 6.19.8.
- Diodorus Siculus, 4.26.2.
- Photius, Bibliotheca excerpts, 190.38
- Hyginus Theoi.com: Hyginus, "Astronomica 1" translated by Mary Grant