Different summons consume different shares of experience.
The C4 revision sets Kai the Cat, Unicorn Merrow and Soulless to 10% XP consumption; Reanimated Man, Dark Panther and Mechanic Golem to 15%. These are named servitor rules, not a universal pet or Sin Eater rate.
C5 applies an unquantified attack penalty when a servitor attacks a player, but not while hunting monsters. This is distinct from its XP consumption. The wording does not establish the same penalty for pets.