When animals die, is there any stage similar to the stage of the grave that humans go through?
Nothing has come to indicate what happens to them after their death except the fact they will be resurrected along with the people.
Ash-Shaykh Muhammad bin Saalih al-Uthaymeen, rahimahullah, said when speaking about a similar topic pertaining to if the angel of death is delegated to take the souls of the animals or not:
في مثل هذه الأمور الغيبية خذ ما ثبت ودع ما لم يذكر … فعلينا يا إخواني! أن نأخذ من مسائل الغيب ما ثبت عندنا ، والباقي نسكت عنه ، لو كلفنا به أو كان لنا مصلحة في معرفته لبينه الله
“… In the likes of these matters of the unseen you should take what has been affirmed and you should leave off what hasn’t been mentioned… Therefore it is upon us oh fellow brothers that we take from the matters of the unseen what has been confirmed for us and we should keep silent for what remains [of what wasn’t mentioned to us]. If we were held responsible for it or if there was some benefit for us in knowing it, Allah would have clarified it for us….”
AbdulFattaah bin Uthman