Is it permissible to plant an olive tree on the grave of the deceased, whether human or animal?

Planting trees on graves: If it is intended as an act of worship, then it is a forbidden innovation (bid'ah) because it was not part of the guidance of the Salaf (pious predecessors). However, if it is intended for the shade of visitors or for any other good purpose, then there is no harm in it, and the doer may be rewarded.

As for burying animals: There is no harm in burning or burying them if it is to prevent the spread of infectious diseases. The ruling on planting trees over them is the same as the ruling mentioned previously: if it is for worship, then it is an innovation (bid'ah), and if it is for other purposes, then its ruling depends on the intention.