Three of the following four are alike in a certain way and thus form a group. Which is the one that does NOT belong to that group? (NOTE: The odd one out is not based on the number of vowels or consonants in the word)

Four letter-pairs are given below. The letters on the left side of (-) are related to the letters on the right side following a specific logic/rule. Three of them share the same rule. Select the odd one out.

Four letter-pairs are given. The letters on the left side of (-) are related to the letters on the right side of (-) by some logic. Three of them follow the same logic. Select the odd one out.