Step 1: Examine the letter pattern.
\[
TUB,\ SON,\ RIN,\ CAT,\ SUN,\ DOG
\]
All these words follow the pattern:
\[
\text{Consonant - Vowel - Consonant}
\]
Step 2: Check AIR.
\[
AIR = \text{Vowel - Vowel - Consonant}
\]
Thus AIR differs from all the remaining words.
\[
\boxed{AIR}
\]