This was a word I had not heard until recently. An endling is the final individual of a species. When the endling dies, the species dies with it, and becomes extinct. Other words that were suggested for this were ender, terminarch, or relict. How lonely it must be to be an endling.
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So sad that endlings have no mate and when they die the species is extinct. We should be able to do more for them because every creature deserves to continue.