Type/Species: Great Serpent
Origin: Egyptian Mythology
In Egyptian mythology, Mehen was primarily the subject of myth but not necessarily an object of worship.  However, the earliest references to Mehen come from the Middle Kingdom's Coffin Texts, which reference the 'mysteries of Mehen.' It is possible that this phrase relates to the veneration and worship of the serpent deity,  although later, people of the New Kingdom spoke of her extensively in vignettes in the Underworld Books. 
Mehen received special mention in the description of the Thephet-Shetat Division of the Kingdom of Osiris: 
On the left of AFU-RA we have also a number of gods and goddesses who belong to this division, and among them may be specially noticed the serpent Mehen, the lord of this region; his body is bent in the form of a canopy, and beneath is the 'Flesh of Osiris.'
-- The Egyptian Heaven and Hell, Seventh Division of the Tuat 
Mehen as the Protector of Ra
Mehen, the coiled serpent god,  was an important guardian deity. Each night as Ra the sun god traveled through Tuat, the underworld  on his journey from the west to the east horizon on the sun boat, named Mesektet, which was protected by Mehen, who coiled around it for its defense against the monster Apophis. 
Her protective cobra hood was often referenced: "...Ra, who stands under a canopy formed by the body of the serpent Mehen..."  Likewise, her coils were also praised: "He stands within a shrine which is enveloped in the voluminous folds of the serpent Mehen, a serpent also stands on his tall before him." 
Excerpts of Mehen's journey could be found in various texts:
This is the Mehen serpent of the uraei, which strideth through the Tuat.
-- The Book of Gates, Chapter XII: The Gate of Sethu 
This god maketh his way into this City, being towed along by the gods of the Tuat, in his hidden form of Mehen. This god sendeth forth a cry to the regions of every Circle of this City, and also to the gods who are therein, and it is the voice of them which this god heareth after he hath sent forth a cry to them.
-- The Book of Am-Tuat, Chapter VIII: The Eight Division of the Tuat 
The gods pulled the sun boat at the behest of Mehen.  In The Book of Am-Tuat, Mehen and the deities were described by this excerpt:
Those who are in this picture are in front of this great god, and they carry the serpent Mehen-ta on their heads into this City, and they travel onwards in the following of Ra into the Eastern Horizon of the sky. This god crieth unto them by their names, and he decreeth for them what they have to do. And Ra saith unto them:
'O ye who keep ward over your serpent-figures with your two hands, lift ye up your heads, whose hands are strong, whose feet are firm, who perform the journeyings which ye are bound to make, who make long your steps as ye go, unite ye yourselves to your offerings in the Hall of the Eastern Horizon.'
Their work is to make the serpent Mehen to travel to the Eastern Hall of the Horizon, and they unite themselves to their habitations after this great god hath passed through the darkness and hath taken up his place in the horizon.
-- The Book of Am-Tuat, Chapter XI: The Eleventh Division of the Tuat 
In addition to his guardianship of the solar boat, some texts attribute Mehen as the guardian of Osiris's corpse. 
In the Hymn to Amen-Ra, Mehen is mentioned as the serpent goddess.  The Coffin Texts claim Mehen as the warden of criminals, who primarily were enemies of Ra; thus, the spells calling on Mehen in the name of deceased criminals assisted in the serpents work (protecting Ra).  She is also mentioned in the First, Seventh, and Eighth Hour of the Tuat. 
Prayers that mention Mehen
Mehen was mentioned in many prayers to or for Ra. Some are included below:
The Majesty of this great and holy god saith, Grant thou me to come forth on the path by thy spittle and by [thy] throat and let me utter the word which is maat to Ankhit, and let me open thy fold, for I have come to illumine the darkness, and to embrace him that is in Mehen.
-- Chapter VII: The Seventh Division of the Tuat 
This god saith unto Osiris, who dwelleth in the serpent Mehen, Hail, Osiris, Governor of the Tuat, thou lord of life, thou ruler of Amentet, thou shalt live, live thou life, thou hast magical power, and shalt prevail by magical power in [this] land... O let me pass over thee in peace.
-- Chapter VII: The Seventh Division of the Tuat 
Let there be stability to the Disk of Ra. Let him that is in the shrine have the mastery over it, and let the serpent [Mehen] guard him well. May the flames of Khuti which are in the corners of the hidden shrine grow stronger.
-- The Book of Gates, Chapter III: The Gate of Saa-Set 
The Egyptian Underworld Books (such as The Book of Am-Tuat) depict Mehen as an immense serpent coiled around or above the cabin of the solar barque, protecting Ra. 
- Mehen protected the Osiris's body after his death. 
- Mehen guarded the solar boat that carried Ra, the sun god, through the underworld. 
- She was the warden of criminals. 
- She was considered a sort of counter Apophis. 
- Rose [Dragons] 241
- Turner [Ancient] 315
- Pinch 200
- Cotterell 218
- Wilkinson [Complete] 223
- Wilkinson [Complete] 224
- The Book of Am-Tuat, Chapter VIII
- The Book of Am-Tuat, Chapter XI
- The Book of Gates, Chapter XII
- The Book of Am-Tuat, Chapter VII
- The Egyptian Heaven and Hell, Seventh Division of the Tuat
- The Book of Gates, Chapter III
- The Egyptian Heaven and Hell, Chapter I
For more information on footnotes and references, please see the bibliography.