Can we say "May Allah have mercy on you" for dead animals?

Mercy towards animals and kindness to them during their lives are encouraged, as stated in the Hadith: "Those who show mercy, the Most Merciful will have mercy upon them. Show mercy to those on earth and the One in the heaven will have mercy upon you," and "If you show mercy to a sheep, Allah will have mercy upon you." As for supplicating for mercy for animals after their death, we do not know of any Sunnah or tradition from the the predecessors concerning it. An animal is not held accountable after its death, except for injustices that prayers for mercy cannot expiate.