Four numbers have been given, out of which three are alike in some manner and one is different. Select the one that is different.

256 144 81 676

81

256 - 162 (  Even number square that is 16) 144 - 122 (  Even number square that is 12) 81 - 92 (  odd number square that is 9) 676 - 262 (  Even number square that is 26)