[Avodah] Is it Assur to eat Neveilah?

[Akiva Miller via Avodah]

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R' Meir G Rabi asked:

> It cannot be Shechted to prevent it being a Neveilah, just
> like a horse for example, cannot be Shechted.

Why can't a horse be shechted? Is it missing the simanim?

Of course, it would still be assur to eat the horse because it is a
tamay species, but would that prevent one from doing a valid shechita?
(I vaguely recall a case where a choleh sheyesh bo sakana was
prescribed pork by his doctor, and the rav told him to shecht the
chazir.)

Akiva Miller