Descriptions
 Hesiod's Echidna was half beautiful maiden (presumably the upper  half) and half fearsome snake. Hesiod described "the goddess fierce  Echidna" as a flesh eating "monster, irresistible", who was like neither  "mortal men" nor "the undying gods", but was "half a nymph with  glancing eyes and fair cheeks, and half again a huge snake, great and  awful, with speckled skin", who "dies not nor grows old all her days."
[9]  Hesiod's apparent association of the eating of raw flesh with Echidna's  snake half, suggests that Hesiod may have supposed that Echidna's snake  half ended in a snake-head.
[10] And 
Aristophanes,  who makes Echidna a monster of the underworld, gives her a hundred  heads (presumably snake heads), matching the hundred snake heads Hesiod  says her mate 
Typhon had.
[11]
 In the Orphic account (mentioned above) Echidna is described as  having the head of a beautiful woman with long hair, and a serpent's  body from the neck down.
[12] Nonnus, in his 
Dionysiaca, describes Echidna as being "hideous", with "horrible poison".
[13]
 Offspring
 According to 
Hesiod's 
Theogony,  the "terrible" and "lawless" Typhon, "was joined in love to [Echidna],  the maid with glancing eyes" and she bore "fierce offspring".
[14] First there was 
Orthrus, the two-headed dog who guarded the Cattle of 
Geryon, second 
Cerberus, the multiheaded dog who guarded the gates of 
Hades, and third the 
Lernaean Hydra, the many-headed serpent who, when one of its heads was cut off, grew two more.
[15] The 
Theogony may also have given Echidna as the mother of the 
Chimera, a fire-breathing beast that was part lion, part goat, and had a snake-headed tail,
[16] though possibly the Hydra or even Ceto was meant as the mother of the Chimera instead.
[17] Hesiod next mentions the 
Sphinx and the 
Nemean lion  as having been the offspring of Echidna's son Orthrus, by another  ambiguous "she", read variously as the Chimera, Echidna herself, or even  Ceto.
[18] In any case, the 6th century BC 
lyric poet Lasus of Hermione, has Echidna and Typhon as the parents of the Sphinx,
[19] while the 5th century BC playwright 
Euripides, has Echidna as her mother, without mentioning a father.
[20] To this list of offspring of Echidna and Typhon, the 6th century BC mythographer 
Acusilaus (along with mentions of Cerberus and "other monsters") adds the 
Caucasian Eagle, that every day ate the liver of 
Prometheus,
[21] and Pherecydes also names Prometheus' eagle, and adds 
Ladon, the dragon that guarded the 
golden apples in the 
Garden of the Hesperides (according to Hesiod, the offspring of Ceto and Phorcys).
 Later authors mostly retain these
 offspring of Echidna and Typhon while adding others. 
Apollodorus, in addition to having as their offspring Orthrus, the Chimera (citing Hesiod as his source), the Sphinx, the 
Caucasian Eagle, Ladon, and probably the Nemean lion (only Typhon is named), also has the 
Crommyonian Sow, killed by the hero 
Theseus (unmentioned by Hesiod). 
Hyginus  in his list of offspring of Echidna (all by Typhon), retains from the  above: Cerberus, the Chimera, the Sphinx, 
the Hydra and Ladon, and adds  "Gorgon" (by which Hyginus means the mother of 
Medusa rather than Hesiod's three 
Gorgons, daughters of Phorcys and Ceto, of which Medusa was one), the 
Colchian Dragon that guarded the 
Golden Fleece[26] and 
Scylla.
 
Nonnus makes Echidna the mother of an unnamed, venom spitting, "huge" son, with "snaky" feet, an ally of 
Cronus in his war with 
Zeus, who was killed by 
Ares. The sea serpents which attacked the Trojan priest 
Laocoön, during the 
Trojan War, were perhaps supposed to be the progeny of Echdna and Typhon. Echidna was also supposed to be the mother by 
Heracles, of Scythes, an eponymous king of the 
Scythians, along with his brothers 
Agathyrsus and 
Gelonus (see below).
 
Cave
 According to Hesiod, Echidna was born in a cave, and lived alone (in  that same cave, or perhaps another) "beneath the secret parts of the  holy earth ... deep down under a hollow rock far from the deathless gods  and mortal men", a place appointed by gods, where she "keeps guard in  Arima".It was from this cave, presumably, as Apollodorus tells us, that she used to "carry off passers-by".
 Hesiod does not say where Arima might be, but presumably, it is the same place where, according to 
Homer, Zeus fought Typhon, which Homer describes as "the country of the Arimi [
Ἀρίμοις], where men say is the couch [bed] of Typhoeus", Typhoeus being another name for Typhon. The late sixth century early fifth century BC poet 
Pindar has Typhon born in 
Cilicia,and has Zeus slaying Typhon "among the Arimoi",
] and the fourth century BC historian 
Callisthenes located the Arimoi and the Arima mountains in Cilicia, near the 
Calycadnus river, the 
Corycian cave and the Sarpedon promomtory.
 Quintus Smyrnaeus locates her cave "close on the borders of Eternal Night".