Zdanie z powieści

Nie jestem pewien jak przetłumaczyć to zdanie (szczególnie jego końcówkę);
"They were what the species he'd once belonged to could not bear to be".

Moze to Ci pomoze:
"They were, what the species he'd once belonged to, could not bear to be".

Napisz jak to rozumiesz po polsku.

or paraphrasing can help

They (absolutely) didn't resemble the species he had once belonged to. They were different
but he couldn't quite figure out what they really were.
Does it make sense in your context?

This is a perfect example of a pushdown nominal relative clause
functioning indirectly in the head clause as part of another clause element.

they were the things that his previous species hated to become.