It’s a DMR STS tensioner.
It sits on the skewer, and yeah, it bolts onto the hanger. The angle of the photo maybe makes it look a bit more exotic that it is … 😕
There were (maybe still are) lots of negative reviews around, as the bolt (for the hanger threads) was (and maybe still is) made of comedy metal, and rather than just taking account of this and doing it up appropriately / sensibly, there seems to have been a number of ham-fisted monkeys DIY mechanics that have assumed that because it’s a big bolt, you need that bit of scaffolding pipe on the end of the spanner to get it done up, which perhaps unsurprisingly … wasn’t supported by the outcome (which is to say there were a lot of rounded off hex-sockets in the bolt heads).
Mine went on ages ago and has only been tweaked once, and even then, I don’t think it’d moved, it was one of those … “I wonder if this need’s tightening?” moments when I was at that end of the bike with the right sized allen-key in my hand.
(For completeness, the rest of it is: XT (non-disc) hub, 20t Surly cog on the freehub body, spaced out by Velosolo spacers, and a 3/32 gusset half-link chain (which might actually be fitted the wrong-way ’round – Clearly I’ve not died as a consequence tho’ as I assume it must be ok like that 😉 )).