Mantelopes

Giraffe-like posthumans who retained their intelligence, but were given near-useless bodies to use it with.

• And I Must Scream: The Mantelopes were transformed into quadrupeds but were left with human minds, used as singers and scribes by the Qu. When the Qu left, the Mantelopes were unable to control their environment in any way. They built a culture of pain around their impotence and ennui before devolving into dumb animals.

• Blessed with Suck: They retained their sapience... with bodies that were unable to take advantage of it. The only thing their intelect was capable of doing was making them realize the helpless state they were put in.

• Despair Event Horizon: A species-wide example, their inability to control the environment led them to simply giving up on trying to do anything and their culture was entirely based upon this pain and suffering, leading them to losing their sapience eventually.

• Mercy Kill: A non-fatal example, the author ponders that their lost sapience might have been a merciful fate for them given their helpless state and inability to do anything with their intellect.

• Uncertain Doom: It's unclear whether they evolved into some new non-sapient species or if they simply went extinct after losing their sapience.