(I'll try to gather all reported info into this post...)
Reported behavior:
Hounds can appear in both quests and wilds. They can "wait" (for an unknown time, but at least a minute) outside a Wilderness quest entrance. Hounds spawn up out of the ground. They spawn "on the trail" of their target, i.e. where they have been (or where they currently
are if they have stopped long enough at the wrong time).
There is a howl that all can hear. Some time later (tbd, est 20-30 sec?) a random(?)* party member becomes "hunted". For a second or three there is a blue arrow pointing down at the target, and they receives a debuff "Target of the Hunt", visible in their de/buff bar at the top of the screen.
(* Several times, the "last" party member was observed to be targeted, someone who had fallen behind the main group - but it's not clear if that was a factor in the target selection.)
Hunting Hounds are strong and tough, but they are not fast - they can be kited fairly easily (observed w/ standard +25% run boost from
Anger's Step). However, they
will teleport to their target if it is "too far" away, similar to some Reapers.
It's possible to have Champion Hounds, which only makes them worse. Champ Hounds can drop rems (rem chests have not (yet?) been reported).
It's possible to pull the aggro off the original target.
Any party member can also receive an appropriate "elemental" debuff, stack of
''X Vulnerability, +20% damage'' - and it stacks 20x!
Estimated cooldown between hunts ~10 minutes (unconfirmed).
(more info as it comes in...

)
Suggested Tactics:
1) Stay with the herd - er, stay with the group! Strength in numbers. Do not zerg ahead, and
definitely do not fall behind!
2) Keep your head on a swivel - keep looking behind the group, anywhere you've been,
especially right after you hear The Howl.
3) Apparently(?), these things do not scale with party size - do not solo, try to quest only with a (nearly) full party.
4) Treat the hound as a dangerous Champion or Reaper - kill it first!
5) If you hear a howl, let the party know. If you are Hunted, let the party know!
6) If you observe party members who do not "help" Hunted members - it's your call what action to take, but consider your options.
Good luck!