…. I’m pretty certain I saw it brought down in R’ Bachaya but maybe not in Ki Siso.
In the meantime, I found this :
The divine kabbalist, the Rama of Pano, presents a tremendous chiddush in Asarah Maamarot concerning the shape and image of the eigel. The source for his chiddush is the Tikunei Zohar (Tikun Sheini 5, 142a). The Rama of Pano writes: “אמרו עליו באגדה, שמחציו כלפי ראשו היה כתבנית שור אוכל
עשב, וכלפי זנבו היה חמור, והן שתי מדות בקליפה… ועל כלאים זה אמרו אלה אלהיך
ישראל”. They stated in the Aggadah that its upper half, toward its
head, resembled an ox, eating grass; its lower half, toward its tail, resembled a donkey. These represent two elements of the klipah . . .