I think DL is one of those where you can get listed as multiple positions:
Pro Football Reference has him as a DT
Haloti Ngata NFL & AFL Football Statistics - Pro-Football-Reference.com
But if you look at his years chart:
2006 22 BAL RDT
2007 23 BAL RDE
2008 24 BAL NT
2009 25 BAL LDE
2010 26 BAL LDE
Pretty much can play anywhere I would say.