As per this chapter of UTTARA KANDA of Valmiki Ramayana:
11 नन्दीश्वरम अथापश्यद अविदूरस्थितं परभुम
दीप्तं शूलम अवष्टभ्य दवितीयम इव शंकरम12 स वानरमुखं दृष्ट्वा तम अवज्ञाय राक्षसः
परहासं मुमुचे मौर्ख्यात सतॊय इव तॊयदः13 संक्रुद्धॊ भगवान नन्दी शंकरस्यापरा तनुः
अब्रवीद राक्षसं तत्र दशग्रीवम उपस्थितम14 यस्माद वानरमूर्तिं मां दृष्ट्वा राक्षसदुर्मते
मौर्ख्यात तवम अवजानीषे परिहासं च मुञ्चसि15 तस्मान मद्रूपसंयुक्ता मद्वीर्यसमतेजसः
उत्पत्स्यन्ते वधार्थं हि कुलस्य तव वानराः
There Nandi stationed at the side of that deity, suppoting himself on his flaming dart, resembling a second Sankara.
And seeing that one having a monkey like face, the Rakshasa, deriding him, burst out into laughter, and seemed as if a mass of clouds were sending roars.
Thereat, growing enraged, the reverend Nandi Shankara's other body spoke unto that Raksha Dasagriva present there;
As O Dasanana, deriding me for my monkey like form, thou hast indulged in a laughter resembling the bursting of thunder, so Vanaras endowed with prowess, and possessed of my form and energy shall be born for compassing the destruction, of thy race.
Is Nandi's face is depicted as Vānara in any other scripture?