An ox = two eels theory: The Japanese syllabary characters meaning an ox ("うし"), which were written on something with a brush and India ink, looked like two eels, so they became the origin of the custom, this theory states.
丑＝鰻二匹説-平仮名で墨汁を使って毛筆で書いた「うし」と言う文字が、まるで二匹の鰻のように見えたからと言う説。