Answered by Shaykh Faraz Rabbani
Should we recite ”Inna Lillaha Wa Inna Ilayhi Raaji’oon” [‘We are Allah’s and to Him shall we return’] when passing by a deer that has died?

Walaikum assalam wa rahmatullah,
It is not specifically recommended for other than humans, but one would be rewarded if one said it.
Wassalam,
Faraz Rabbani