Let me make sure I understand you:
So in this case, because PostID=103 has Type=101, you would want to show only PostID=103, because it is "later" (higher number) than 101??
PostID ThreadID Type
101 37 4
102 38 5
103 43 101
You could also do
FROM table AS T1 LEFT JOIN table AS T2
ON T1.Type = T2.PostID
WHERE T2.PostID IS NULL
ORDER BY T1.PostID DESC
But usually the JOIN will be faster, esp. with MySQL.
SELECT * FROM table
WHERE PostID NOT IN ( SELECT Type FROM table )
ORDER BY PostID DESC
It would help performance if you added an index (even a non-unique index) to the Type field.
If I misunderstood you, please post again but show some sample data with expected results.