The idea that HTC would be working on an HD8 is probably would not be too surprising--after all they do have an HD2 and HD7 (really it was the HD3, renamed at the last minute). So in maybe it's not that odd that we caught an HD8 showing up in some app stats recently.
By it's name alone one would think that this was destined for "Apollo" aka Windows Phone 8. However, the information we have right now via device statistics from "...I'm a WP7!" says that it is only running 7740 and it first appeared on December 30th, 2011. This could hint to some real early testing by HTC of this hardware.
We also know it is on a Romanian network (Vodafone.RO) which at first sounds odd until you remember that the recent leak of the LG Fantasy was purchased from a tester-developer in, you guessed it, Romania (and one of first pics of "Tango" came from Slovakia).
Still, we're treating this one with caution but as we mentioned earlier...this is HTC and an HD8 seems like a logical continuation of that series. Hopefully we'll find out more soon.
You can pick up "...I'm a WP7!" here in the Marketplace