Sudden Retreat◆◆ The hippocampus makes a tail Strike, then Swims with a +10-foot circumstance bonus to its swim Speed. It gains a +2 circumstance bonus to AC against reactions triggered by this movement.
Source: Pathfinder Monster Core
Running this fight? Add the hippocampus to the initiative tracker: type its name and the AC, hit points and Perception for initiative fill in, and the book button shows this stat block mid-fight.