Oddworld: Soulstorm Enhanced Edition

This was originally bugged and not unlocking, but was patched on 12/16. Note that the requirements here are stated differently than the PS version, which was based on getting 80% of Mudokons saved in at least 12 of the first 15 levels in order to open up the final 2 levels and get the "good" ending. Saving 1,000 Mudokons is different, in that there are a handful of levels that would matter a great deal and some that wouldn't matter at all since they have relatively very few Mudokons to save.

Here are the levels and how many Mudokons are available to save in each one:
1: 'The Raid on Monsaic ', 300 (All automatically saved)
2: 'The Ruins', 200
3: 'The Blimp', 1
4: 'The Funicular', 24
5: 'Sorrow Valley', 31
6: 'Phat Station', 32
7: 'The Hijack', 15
8: 'Reunion at the Old Trellis', 254
9: 'Slig Barracks', 45
10: 'Necrum', 32
11: 'The Mines', 47
12: 'The Sanctum', 4
13: 'Escape', 414
14: 'Feeco Depot', 25
15: 'The Yards', 27
16: 'Brewery', 81
17: 'Eye of the Storm', 50
Total: 1,582

To see how many you saved in a level afterward, go to level select and view the % saved next to your Quarma icon. Note that this won't always correspond to the number you see in the "Saviour" medal in the detailed stats for the level, because the medal is often given at a lower number than 100% of the Mudokons saved. For instance, whether you saved 50 Mudokons in the final level or only 45, the medal progress will show 45/45; but the % saved will show 90% at 45 and 100% at 50.

I believe that if you already finished the game, you'll have to replay it to get the achievement.