The 18-year-old previously spent two years with the Chairboys youth team before spending last season with Boro.
The shotstopper left Graham Westley's men following the completion of his apprenticeship, paving the way for him to commit himself to the Chairboys for the 2013/14 campaign.
"I'm over the moon," Horlock told the club's official website. "It's brilliant to be coming back and I know from last time that there's a great group here. I'm so happy."
Manager Gareth Ainsworth added: "He's done nothing but impress me since he was at the club first time round. It's great to have him here and he's a great character.
"His work-rate and attitude is superb for such a young man. He wants to learn and be around the place.
"To have players like him around place is infectious and the more you have, the better it is for the club."