mərəγō saēnō ‘the bird Saēna’

Khorda Avesta, Rashnu yasht (yasht 12) §17 (awending James Darmesteter):

'ýatcit ahi rashnvô ashâum upa avãm vanãm ýãm saênahe ýâ hishtaite maidhîm zrayanghô vourukashahe ýâ hubish eredhwô-bish ýâ vaoce vîspô-bish nãma
ýãm upairi urvaranãm
vîspanãm taoxma nidhayat
zbayamahi ....

 

Whether thou, O holy Rashnu! art on the tree of the eagle, that stands in the middle of the sea Vouru-Kasha, that is called the tree of good remedies, the tree of powerful remedies, the tree of all remedies, and on which rest the seeds of all plants; we invoke, we bless Rashnu, the strong. I invoke his friendship towards this var prepared....

See also Yasht 14.41.

śyeno of Rigveda 4.18.13, where Indra (their evenworth of our Thunor/Thur, the Northern Thor), is made to say:

avartyā śuna āntrāṇi pece na deveṣu vivide marḍitāram |
apaśyaṃ jāyām amahīyamānām adhā me śyeno madhv ā jabhāra ||

In deep distress I cooked a dog's intestines. Among the Gods I found not one to comfort.
My consort I beheld in degradation. The Falcon then brought me the pleasant Soma.

 

From Avestan mərəγō saēnō ‘the bird Saēna’, through Pahlavi Sēnmurw cometh simorḡ.  Thus the arn in the tree becometh the simurgh, see Zādspram's Anthology (Vizīdagīhā ī Zādspram) chap. 8 (awending E. W. West):

The fourth battle, that of the plants

1. As he (Ahriman) came fourthly to the plants -- which have struggled (kukhshi-aito) against him with the whole vegetation -- because the vegetation was quite dry, Amurdad, by whom the essence of the world's vegetation was seized upon, pounded it up small, and mixed it up with the rain-water of Tishtar. 2. After the rain the whole earth is discerned sprouting, and ten thousand special species and a hundred thousand additional species (levatman sardako) so grew as if there were a species of every kind; and those ten thousand species are provided for keeping away the ten thousand diseases.

3. Afterwards, the seed was taken up from those hundred thousand species of plants, and from the collection of seed the tree of all germs, amid the wide-formed ocean, was produced, from which all species of plants continually grow. 4. And the griffin bird (simurgh) has his resting-place upon it; when he wanders forth from within it, he scatters the dry seed into the water, and it is rained back to the earth with the rain.

5. And in its vicinity the tree was produced which is the white Haoma, the counteractor of decrepitude, the reviver of the dead, and the immortalizer of the living.

6. This was the fourth contest, about the plants.