One thought: maybe the gloomfetch can generate a kind of permanent "reverse shield other" effect (something like a shield me spell, if such a thing existed), such that half of any damage sent its way (by deliberately targeting the head) is transferred to the thrall, whereas any non-deliberately-targeted attacks automatically get applied to just the thrall. (Attacks that affect an area would naturally apply to both.) It just makes sense that a gloomfetch would rather shunt as much damage as it could to its disposable (and replaceable) thrall.
So the gloomfetch only takes damage applied to a whole area (like an explosion) or specifically targeted at the head, and even then half of the damage gets sent to the thrall instead. I'd like that deal if I were a gloomfetch.
Johnathan