Horton, who has more than 1,000 games as a boss on his CV, managed Manchester City in the Premier League and was Phil Brown's assistant when Hull City won promotion in 2008.
The 64-year-old will support new Rovers boss Dickov, who replaced Brian Flynn last month following the club's promotion to the Championship.
He told the club's official website: "It's brilliant to be here. It's a great club with a fantastic training ground and stadium, and with promotion to the Championship last season I'm really looking forward to it.
"I was asked by Phil Brown whether I would go to Southend, which I chose not to do. Paul then asked me what I was doing and whether I would be interested in joining him here if he got the job.
"We sat down and had a chat and discovered we have the same footballing ideas so I'm delighted to be here with him.
"I know Paul through the Manchester City connections and when we spoke he said he wanted some experience with him and that is why I decided to come - it's great to be back in football."
Dickov said: "Brian is someone who will be fantastic for me having managed over 1,000 games and probably had around 600 or 700 games as assistant manager."