On higher pirates difficulties they can spawn behemots as part of theirs fleets. I think it starts with hard lvl.

Ok, that's interesting. I am playing on Endless difficulty. That's probably it.

I could be wrong, but this seemed to spontaneously spawn right on an adjacent system (I had an explorer parked there) and not from their system node, but that's really hard to confirm. I guess I could go back to an auto save and move my explorer along the lines to see if it actually traveled there.

I'm not sure about place of spawn origin, nevertheless never saw behemots in pirate fleet on lower then hard pirate difficulty. Maybe I'm very unlucky but I think they can also spawn as part of the quests but not sure which ones.

So, I destroyed it and got the following immediately. Makes me think that thing is unrelated to pirate spawn, and instead associated with the Hissho faction (at least, I've never seen it happen with another faction in my playthroughs).

(right click image and select "Open image in New Tab" to see larger.)