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Not only that, but a neutrino of that energy! Its speed must have been ridiculously close to c, rather than "only" extremely close to c like almost all other neutrinos.

Is not that a ludicrous speed? :)

It's gone plaid

Another impressive one: Ultra high energy cosmic rays have been detected to 3e20 eV (300 EeV; detected by the Fly's Eye Collaboration). They're nuclei, however, so the rest mess is at least about 10000x higher.

https://en.m.wikipedia.org/wiki/Ultra-high-energy_cosmic_ray

If you're interested in popular science but haven't read about this before, consider following the link to the GZK cutoff from that page. It's super cool.

I used to work for the Pierre Auger Observatory. Happy to try to answer any questions if folks are curious. :)

Isn't the rest mass of a proton at least 8 billion times higher than that of a neutrino?

You're right. Oof, I need to stop reading HN late at night when I can't even read units wrong or have any intuition. Apologies.

Now that would be spooky action at a distance.