Gallic (Continental Celtic) god, part of the trinity of "Taranus, Esus, and Teutates" according to Roman historian Lucan. According to Lucan, Esus' human victims were sacrificed by being tied to a tree and flailed.
Esus has been identified by some with Mercury, others with Apollo. However, a strict identification with a Graeco-Roman god is difficult, if not impossible. French folklorist Jean Markale identifies Esus instead with a type of god--the "horrible" sky god of magic, like Odin (whose victims were also tied to a tree and flailed), Math ap Mathonwy/Gwydion (lords of magic), and Uranus.
Esus has somtimes been written as Hesus, which has lead to some "scholars" to call him "Hu-Hesus" and identify him with Jesus and Hu Gadarn (a somewhat hypothetical Welsh god) together, a rather silly idea.