For the achievement Never Seen That Before, you need to kill one of the listed unique enemies. For the achievement Special Snowflakes, you need to kill 15 of these same listed unique enemies. For the achievement A Unique Collection, you need to kill all of these listed unique enemies. Where are they usually found (what zones, and in what general directions/areas within those zones)? Do some spawn more predictably than others?
There are 82 of these and they are all found randomly. I've added all the ones that I've confirmed personally to start with plus those listed on the Diablo 3 database that I've verified on other sites. For the sake of completeness and simplicity, I'm posting this as a community wiki post so that people can add what they find:
If you edit the lists above, please note the specific area and sub-area where you found them.
Edit: Since skovacs1 posted another list, I am only updating this one with a few that I personally find. I may reorganize this to be more generally useful in future.
I found an incomplete list on the Diablo III database
Everyone questing for the Staff of Herding is going to kill Chiltara, he drops the gibbering gemstone. Another post I read places Beyatt in Black Canyon Mines.
These are all random quest mobs found all over Diablo. They all spawn in their own territories but not always, basically just a random quest generator. There's usually a bit of story or a rare chest and then you kill them as the boss.
Gorog the Bruiser spawns after the Spider Queen in goat territory when you are looking for the staff. I only remember this guy because he had a mechanic I hadn't seen before - if he continuously hit me he hit harder and harder.
I've killed 7 more but don't remember their locations by their name. I know there's one or two in Desecrated Fields.
I haven't directly found the ones missing from the list above, but I have found a few references on other sites and then went through the game strings to narrow down the rest. In the case of strings, I looked the unique up on http://www.d3lexicon.com/, looked at the base of the string, went to that entry to find the enemy's name, then looked up that enemy in D3DB to find the zones it can spawn in. Keep in mind that if D3DB is missing spawn zones for the enemy type, or if the unique appears in a zone where its enemy type does not, then these zones might be a waste of time. (Off the top of my head, I could have sworn Enslaved Nightmares could spawn on Silver Spire level 1 as well, but I can't check right now because of maintenance.) It's also possible some of these also spawn in the more rare dungeons, and only there. But it's a starting point. Please do not assume you will actually find the unique in the zones listed, only use this info if it's the only info you've got.
Edit: I asked the administrator of D3Inferno.com if there is any way to read the spawn zones from the game data, and he told me there is not, because that information is held server-side. So until someone actually finds a unique monster, this is the best we can do.
So if anyone is searching for these, here's what I found:
Gart the Mad, Maniacal Fallen Master - its string is for a Fallen Overseer, which D3DB says can spawn in Act 2 Black Canyon Bridge, Black Canyon Mines, Howling Plateau, Road to Alcarnus, and Stinging Winds.
Grimnight the Soulless, Shadow of Death - its string is for an Enslaved Nightmare, which D3DB says can only spawn in Act 4 Silver Spire Level 2. I thought these could spawn on Silver Spire Level 1 as well, but cannot check at the moment. I have this checked off on my list, I must have killed him before I knew about the achievement...
Hazzor the Viper, The Sibilant - its string is for a Serpent Magus, which D3DB says spawns in Act 2 Dahlgur Oasis, Khasim Outpost, The Ruins Level 1, and The Ruins Level 2. However, I have also seen Serpent Magi spawn in The Ruined Cistern, and The City of Caldeum during the few events that happen there.
Jezeb the Conjuror, Cultist Beast Caller - its string is for the higher level Dark Summoner that D3DB says spawns in Act 1 Cain's House (can probably disregard), Halls of Agony Level 1, Halls of Agony Level 2, Halls of Agony Level 3, Leoric's Hunting Grounds, Leoric's Manor, and Act 2 Sundered Canyon. The post above that says it was encountered in Act 1 Halls of Agony 2 or 3 is likely correct, so check those two levels as that's much narrower.
Kysindra the Wretched, Lust's Forlorn - its string is for a Hell Witch, which D3DB says only spawns in Act 4 Silver Spire Level 1. The above post says "Silver Spire Level 2?" I could have sworn there are succubi in level 2, and if there are they are called Hell Witches as well, which would mean Kysindra could be in level 2, but I will edit after maintenance if it turns out no succubi spawn in level 2. Plus it's always possible Kysindra still just spawns in Level 2, regardless.
Mage Lord Ghuyan, Guardian Lord of Fire - its string is for a Smoldering Construct, which D3DB says can only spawn in Act 2, The Unknown Depths. However, it said the same thing for Mage Lord Skomara, and I found him in the Storm Halls.
Marchocyas, Winged Horror - its string is for a Demonic Hellflyer, which D3DB says can spawn in Act 3 Rakkis Crossing, Skycrown Battlements, Stonefort, The Barracks Level 1, and The Battlefields. However, the post above says "I believe I encountered this in Act 4" which may very well be true.
Rhau'Kye, The Eye of Flame - its string is for a Morlu Incinerator, which D3DB says can spawn in Act 4 Arreat Crater Level 1, The Silver Spire Level 1, and The Silver Spire Level 2.
Saha the Slasher, Infected Terrifying Claws - its string is for a Lacuni Huntress, which D3DB says can spawn in Act 2 Black Canyon Mines, Howling Plateau, Khasim Outpost, Stinging Winds, and Sundered Canyon. I'm pretty sure these also spawn in some early random Act 2 dungeons as well, though.
Slarg the Behemoth, Massive Hellbeast - its string is for a Mounted Armaddon, which D3DB says can spawn in Act 4 Gardens of Hope 2nd Tier, Hell Rift, and The Silver Spire Level 1. I could have sworn these can also spawn in Silver Spire Level 2.
Veshan the Fierce, Cloaked by the Unseen - its string is for a Subjugator, which D3DB says can spawn in Act 4 Gardens of Hope 1st Tier, Gardens of Hope 2nd Tier, Hell Rift, and The Silver Spire Level 1. Could have sworn these spawn in Silver Spire Level 2, as well.
Blarg the Imp-spawns in the desolate sands, b/f and I ran into him by accident the other day, on my map he was near the archives at the northern area, but I am not sure he would not be elsewhere given random maps
I found The Crusher in Act III in the Keep Depths. Pretty sure it was on level 2
Gholash is in the secret oasis. he spawsn after you hit bone piles near a little puddle of water (randomly of course)
