Step 1: Compare CAT with DBU letter by letter.
C goes to D, A goes to B, T goes to U.
Step 2: Spot the pattern.
Each letter moves one step forward in the alphabet.
Step 3: Apply it to D.
The letter after D is E.
Step 4: Apply it to O.
The letter after O is P.
Step 5: Apply it to G.
The letter after G is H.
Step 6: Put the letters together.
E, P, H gives EPH.
Step 7: State the answer.
DOG is coded as EPH. \[ \boxed{\text{EPH}} \]