Well recently i downloaded the program called sMorph (by SKU). I found an easy way to find a Display ID without going on a private server as a GM or some long way.
Go to wowhead.com and look up the monster you want. Lets try Hogger.
Type in Hogger and click SEARCH, then click his name. Now you should have a page that looks like this.
Now right click anywhere on the page and click "View Page Source" then press CTRL F which should bring up a find box. Type in "displayid" which should take you the creatures display ID.
Now the number 384 is Hogger's display ID. Now type in 384 into the ID part of sMorph then you make people or urself look like Hogger!