While researching about the phoenix I came across this interesting website that says:
The Phoenix is a unique bird, there may only exist one at a time, which ... legend of Phoenix, the same bird is called Vena in the Rig Veda.
Now the Rig Veda has some interesting things to say about this creature:
1 SEE, Vena, born in light, hath driven hither, on chariot of the air, the Calves of Pṛśni (Prsnigarbha). Singers with hymns caress him as an infant there where the waters and the sunlight mingle.
2 Vena draws up his wave from out the ocean. mist-born, the fair one's back is made apparent, Brightly he shone aloft on Order's summit: the hosts sang glory to their common birthplace.
3 Full many, lowing to their joint-possession, dwelling together stood the Darling's Mothers. Ascending to the lofty height of Order, the bands of singers 'sip the sweets of Amṛta.
4 Knowing his form, the sages yearned to meet him: they have come nigh to hear the wild Bull's bellow. Performing sacrifice they reached the river: for the Gandharva found the immortal waters.
5 The Apsaras, the Lady, sweetly smiling, supports her Lover in sublimest heaven. In his Friend's dwelling as a Friend he wanders: he, Vena, rests him on his golden pinion.
6 They gaze on thee with longing in their spirit, as on a strong-winged bird that mounteth sky-ward; On thee with wings of gold, Varuṇa's envoy, the Bird that hasteneth to the home of Yama.
7 Erect, to heaven hath the Gandharva mounted, pointing at us his many-coloured weapons; Clad in sweet raiment beautiful to look on, for he, as light, produceth forms that please us.
8 When as a spark he cometh near the ocean, still looking with a vulture's eye to heaven, His lustre, joying in its own bright splendour, maketh dear glories in the lowest region.
The highlighted portions do resonate with him being identified as a beautiful bird but the other verses confuse the plot. Is Vena really related to Phoenix or is it a reference to the rising Sun? Are these highlighted portions explained in detail in some commentary especially why he is called Varun's envoy?