Many modern Hindus, especially in colloquial speech, refer to the vAhana of rudra as nandin. This is actually very erroneous. Technically nandin was always the mighty gaNa of rudra and not the vAhana, which is vR^iSha(bha). Since the vedic period rudra has a vR^iShbha vAhana (the white bull; though in some vedic representations he has a white horse). The bhUta-gaNeshvara nandin was either bull-faced or ape-faced. His ape-faced form is alluded to in the rAmAyaNa (uttara-kANDa and at least some recensions of the sundarakANDa). In the skanda-purANa (one of the early versions) nandin is termed as kapIndra-vadana. In the saurapurANa (42.20) he is termed vAnarAsya. Throughout the rudra saMhitA of the shiva mahApurANa (Venkateshwara Steam Press edition; the rudra saMhitA was the core of the proto-shiva purANa) nandin is only described as a gaNeshvara and not as a vAhana. In the ajita mahAtantra of the saiddhAntika tradition his iconography is described thus:
shailAdistu prakartvayas trinetrash cha chaturbhujaH |
jaTA-makuTa-saMyuktaH shUlAbhaya-karAnvitaH ||
vAme daNDAkShamAlAbhyAM alaMkR^ita-karas tathA |
daMShTrA-karAlavadano hari-vaktro.athava bhavet || AMT 36.349-350
shailAdi (nandin) should be made with 3 eyes and four arms, wearing a crown of dread-locks with a trident and abhaya pose (right hands). In the left hands he holds a rod and a rosary. He should have fierce face with fangs or that of lion.
In terms of real examples of iconography, the real nandin (not the bull) is depicted as:
1) A bull-faced anthropomorphic form (e.g. Madurai mInAkShI-sundareshvara temple or the earlier pallava kailAshanAtha temple)
2) An ape-faced form (e.g. pallava rock temple of Kunnattur and chera temple of Kottukal in Kerala)
3) An entirely anthropomorphic representation (e.g Darasuram airAvateshvara temple)
4) With a lion face with dread locks in Nepali shiva-parivAra sculptures. This adherence to the AMT prescription mentioned above is interesting for today the AMT tradition survives only in the south where the depictions of nandin do not follow its prescriptions.
5) The Kashmirian depictions of maheshvara where nandin appears as the fourth rear face and is typically shown as a fierce figure with a prominent beard. This is laid out in the bhUteshvara mAhAtmyam as:
bhUteshvaraM bhUtapatiM dR^iShTvA martyo vimuchyate ||
[bhUteshvara] is depicted with faces of sharva, nandin, mahAkAla, and umA; having seen bhUteshvara, the lord of the ghosts the mortal is released.