Subhadra is worshiped along with her brothers Krishna (the avatar of Vishnu) and Balarama (the avatar of Shesha-naga).
As such Was she also an avatar of any Goddess?
If so, Which Goddess and what is the source of that information?
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Krishna's sister Subhadra was the rebirth of Madhavi, daughter of the sage Galava. The story is told in the Nagara Kanda of the Skanda Purana. Galava was looking for a husband for Madhavi, so he enlisted the help of Vishnu's bird Garuda to find a suitable husband. Garuda found that only Vishnu had all the qualities of a perfect husband, and asked Vishnu to marry Madhavi. Vishnu agreed, and as soon as she heard Madhavi say on Vishnu's serpent-bed in Shwetadvipa, before the marriage could even take place. Lakshmi was outraged by this, and cursed Madhavi to have a horse head in her next birth. Vishnu softened the curse, saying that because Madhavi had set on the right side of the bed, usually reserved for family members, Madhavi would be reborn as a family member of Vishnu. Here is what this chapter of the Skanda Purana says:
Sri Garuda said: "I have a very close friend, a Brahmana hailing from the family of Bhrgu. The name of his lotus-eyed daughter is Madhavi. A husband befitting her could not be found by that noble-souled one. Therefore, he said to me: 'O excellent bird, bring a husband suitable to her, if you consider me worthy of honour.' Then the entire earth was searched by me for a bridegroom befitting her. But no bridegroom endowed with all good qualities could be obtained. Then, O Lotus-eyed One, you were recollected by me in my mind as a befitting husband for her. You are endowed with all good qualities. Hence, O Lord of Suras, accept her. Hold her hand in marriage. She is very beautiful. I pray to you with this appeal of mine." Sri Bhagavan said: "O excellent bird, bring that lotus-eyed girl here. On seeing her myself, I shall do as spoken by you." ... On being told thus by that powerful Visnu he brought her and also the Brahmana, a descendant of the family of Bhrgu. The Brahmana bowed down to Madhusudana and came to the side of Vishnu, near Garuda, like Lakshmi herself. Due to child-like innocence the blameless beautiful girl sat at the end of the bed to the right of the Enemy of Mura. Thereat, Lakshmi who had the status of the chief queen became extremely furious thinking her to be her co-wife and cursed the girl: "Since, verily in my very presence, O sinful girl, you set aside all bashfulness and joyously took your seat on the bed of my husband, you will become a hideous Ashvamukhi." ... Sri Bhagavan said: "In this matter, O excellent Brahmana, do not be so enraged. Never can inauspiciousness befall those who come to my vicinity. Hence she will not become horse-faced in this birth. Take her and go home. Give her to a desirable person. It is the left side in the bed that has been assigned to wives and the right side to kinsmen who may lie for the nonce. Here, your daughter, O Brahmana, has occupied the spot reserved for kinsmen. So she will be born as my younger sister in the next birth. I will incarnate then on the earth on account of some task of Devas. Since she has been cursed to become a horse- faced one by my wife, I will perform great penance along with her and transform her into one of splendid face[.]
And indeed, Madhavi was reborn as Krishna's sister Subhadra, and she initially had a horse face until Krishna changed her face through Tapasya. Here is what another chapter of the Skanda Purana says:
A third wife, the beloved of Vasudeva was known as Suprabha. Madhavi of her assuming the form of a horse-faced one. On seeing the daughter born with such a hideous form, Suprabha and Vasudeva became grief-stricken.... Lord Vishnu, on coming to know that his sister was like that and the parents were highly distressed, took her with him. Accompanied by Baladeva he quickly went to the holy spot of Hatakeshvara for performing a great penance. Properly engaged in Yajna, he propitiated Brahma through different kinds of Vrata and Dana and propitiations of Brahmanas. At the end of a year, Brahma was pleased with the Sharnga-armed One. He said "I shall grant boons. Request for the desired thing." Vishnu said: "O Lord, this sister of mine is born as a horse-faced one. Let her be one of excellent countenance. This is my desire." Sri Brahma said: "With my favour this good lady shall be one with excellent countenance. She will become well-known by the name Subhadra. A beloved of her husband, she will give birth to a heroic son. A man who devoutly worships her in this form here, O Vishnu, along with you and also this Sirin (Balabhadra) on the twelfth lunar day in the month of Magha with sweet scents, flowers and unguents shall attain undoubtedly what is cherished in his mind. A barren woman or one abandoned by her husband, who devoutly worships her, O Keshava, on the third lunar day, shall become fortunate, happy and blessed with good sons. She will be always rich and prosperous and endowed with all good qualities."
On a side note, the story of Galava, Madhavi, and Garuda is mentioned in this chapter and subsequent chapters of the Udyoga Parva of the Mahabharata, but it doesn't mention Madhavi trying to marry Vishnu or incurring a curse.
Yes, Subhadra is also an avatar.
Subhadra is an amsa avatar of Satarupa, the wife of Svayambhuva Manu.
We can find it in Brahmavaivarta Purana
From the amsa of Kamala, Draupadi emerged out of the fire altar; from the womb of Devaki, Subhadra was born from the amsa of Satarupa.
and also in Garga Samhita
Powerful Vayu will descend as Bhima, Svayambhuva Manu as Arjuna, Shatarupa as Subhadra, and Savita as Karna.