The Hidden Forest

The spell resistance on the mage donor pet seems to be much lower than the Rk 1 water pet, as the water pet seems to be immune to almost any type of spell, while the donor bp pet gets hit by everything (aoe stuns, dispels, etc. that don't affect the other pets).

Would it be possible to bring the donor pet in line with the T8 pets please?

The Tier 8 water pet is immune to all spells. It is one of the perks to that specific pet. The donor pet does not have that ability.