According to some puranas, Nandi was born to sage Shilada, who got him by the grace of Lord Shiva. Lord Shiva blessed the child and declared that he would be worshiped along with Him and become his vaahana (vehicle).
It is to be remembered that in Srimad Ramayana no mention was made about Lord Shiva as God, except the Prakshipta parts here and there, but mention about "Nandiswara" was made in Srimad Ramayana.
When Ravana saw Sri Hanuma and taken aback by his radiance, thought Sri Hanuma as either Nandiswara or Baanasura.
किमेष भगवान्नन्दी भवेत्साक्षादिहागतः || येन शप्तोऽस्मि कैलासे मया संचालिते पुरा | सोऽयम् वानरमूर्तिः स्यात्किंस्विद्बाणो महासुरः || (Sundara Kanda Sarga 50 Sloka 2)
"When Mount Kailasa was moved by me, is it the same revered Nandi , who cursed me long ago and came here now in this form or whether he is Bana the great demon came here in the form of a monkey."
Here, Ravana was referring "Nandi" as "Bhagvan (भगवन्)". Did he say so, because he was one among many Devatas? I think, it was not the case, because even Indra or Vayu was not referred to as भगवन् in Srimad Ramayana.
In Srimad Ramayana Lord Brahma was referred to as "Pitamaha", "Swayambhu", "Brahma" and occasionally as "Bhagwan Brahma". Lord Indra was referred to as "Sakraha", "Sahasraaksha", "Vaasavaha", "Indra".
Then the question arises as to why Sage Valmiki referred "Nandi" as Bhagavan (भगवन्)?
The answer lies in Srimad Ramayana itself in other place.
After his first encounter with Sri Rama in the war, Ravana gets defeated, humiliated and spared of his life by Sri Rama. He comes back to Lanka and thinks as follows:
उमा नन्दीश्वरश्चापि रम्भा वरुणकन्यका || यथोक्तास्तन्मया प्राप्तम् न मिध्या ऋषिभाषितम् | (Yuddha Kanda Sarga 60 Sloka 11)
"What was predicted by Uma, Nandishvara, Rambha the wife of Nalakubara and Punjikasthala the daughter of Varuna has come to pass! The words of the sages never prove false."
In the above situation, Ravana was saying, while referring to Uma, Nandiswara that words of the sages never prove false! So Nandiswara was a Great Rishi, but not a bull, doing austerities in Mount Kailash area, when Ravana tried to move it and invited a curse from Nandiswara.
If we browse through Srimad Ramayana, we will come across the word "Bhagvan ( भगवन्)" in some episodes describing Great Rishis.
1) Dasaratha requests Vasishta and Vamadeva to start arrangements for the anointing ceremony of Sri Rama.
अभिषेकाय रामस्य यत्कर्म सपरिच्छदम् || तदद्य भगवन् सर्वमाज्ञापयितु मर्हसि | (Ayodhya Kanda Sarga 3 Sloka 6)
"Oh Divine Vasishta! Be pleased to order now for the traditional ceremonial that is to be done and various accessories needed for the anointing ceremony of Rama."
2) Sri Rama, Seetha and Lakshmana reach the hermitage of Bharadwaja, situated at the confluence of Ganga and Yamuna rivers. After entering his hermitage Sri Rama addresses Sage Bharadwaja as follows:
न्यवेदयत च आत्मानम् तस्मै लक्ष्मण पूर्वजः | पुत्रौ दशरथस्य आवाम् भगवन् राम लक्ष्मणौ || (Ayodhya Kanda Sarga 54 Sloka 13)
Sri Rama (the elder brother of Lakshmana) introduced himself to the sage as follows: "Oh! venerable sage! We both are Rama and Lakshmana the sons of Dasaratha."
3) While roaming in the Dandaka Forest, Sri Rama, Lakshmana, and Seetha enter the hermitage of Sage Agastya. Sri Rama informs the greatness of Sage Agastya to Lakshmana and falls on the feet of sun-like radiant sage Agastya, touching them reverentially.
अब्रवीत् वचनम् वीरो लक्ष्मणम् लक्ष्मिवर्धनम् || बहिर् लक्ष्मण निष्क्रामति अगस्त्यो भगवान् ऋषिः | औदार्येण अवगच्छ्हामि निधानम् तपसाम् इमम् || (Aranya Kanda Sarga 12 Sloka 22)
On seeing the brilliantly glowing sage among those sages, Sri Rama told Lakshmana, "Here comes the godly sage, Lakshmana, by his eminence I comprehend him as a depository of all ascesis."
So Nandiiswara was a great Rishi, residing in Mount Kailasa area, but not the divine vehicle of Shiva.
Am I right?