Red Bull designed four different specifications of its 2016 chassis due to uncertainty over its engine supplier next year.
Red Bull is set to continue with Renault engines next season, although details of the arrangement have yet to be confirmed. Initially the team wanted to drop the French manufacturer for Mercedes, but the deal was not finalised and the world champions ultimately opted not to supply one of its main rivals with its prized asset.
Red Bull then went to Ferrari but demanded engine parity with the works team, which Maranello was not willing to grant. Finally it approached Honda before McLaren boss Ron Dennis vetoed the arrangement, leaving Renault as the only viable option for 2016.
The uncertainty means Red Bull is already on the back foot for next season, but Horner said the team had been working so that it could accommodate any one of the four engines in the back of the RB12.
"We've designed four different variants of it," he said. "Inevitably that has put some delay into the process, but with next year being relatively static in rules it is very much an evolution. For sure it has made the timescales much shorter, but the whole team in Milton Keynes has done a superb job in working tremendously hard and long hours to try and get us back on to schedule."
Asked to reveal more details about the 2016 engine contract, he added: "We have got a signed contract. There has obviously been an awful lot of activity over the last few weeks. I am not at liberty to say what that activity is. But I am sure it will all come out in the wash."
Horner confirmed the new deal only applied to Red Bull and not junior team Toro Rosso, which may end up running year-old Ferrari engines.