What is "hippocrene"

Hippocrene \Hip"po*crene\, n. [L., fr. Gr. ?; "i`ppos horse + ? a fountain.] A fountain on Mount Helicon in B[oe]otia, fabled to have burst forth when the ground was struck by the hoof of Pegasus. Also, its waters, which were supposed to impart poetic inspiration.
--Keats.

Nor maddening draughts of Hippocrene.
--Longfellow.

fount on Mount Helicon, sacred to the Muses, from Greek Hippokrene, literally "horse's fountain," from hippos "horse" + krene "fountain."

In Greek mythology, Hippocrene (Ἱππου κρήνης) was the name of a spring on Mt. Helicon. It was sacred to the Muses and was formed by the hooves of Pegasus. Its name literally translates as "Horse's Fountain" and the water was supposed to bring forth poetic inspiration when imbibed.

Hesiod refers to the horse's well on Helicon in his Theogony.

And after they have washed their tender skin in Permessus or Hippocrene or holy Olmeidus, they perform choral dances on highest Helicon, beautiful, lovely ones, and move nimbly with their feet.

John Keats refers to Hippocrene in his poem Ode to a Nightingale.

O for a beaker full of the warm South
Full of the true, the blushful Hippocrene,
With beaded bubbles winking at the brim,
And purple-stained mouth;
That I might drink, and leave the world unseen,
And with thee fade away into the forest dim:

Henry Wadsworth Longfellow mentions the fountain in his poem "Goblet of Life":

No purple flowers,--no garlands green,
Conceal the goblet's shade or sheen,
Nor maddening draughts of Hippocrene,
Like gleams of sunshine, flash between
Thick leaves of mistletoe.

Petrarch refers to the fountain of Helicon in his epic poem Africa

Sisters who are my sweet care, if I sing to you of wonders, I pray that it be granted to me to drink again at the fountain of Helicon.