"We’ve entered the world championship, we’ve signed a contract for an engine, but I can’t tell you what it’ll be or called at the moment," he said at the season-ending Abu Dhabi Grand Prix.
The engine is almost sure to be a Renault one, although likely under another name and possibly with Red Bull's own technical input.
Renault are Red Bull's current partners but the relationship has strained to breaking point, despite four successive drivers' and constructors' titles together from 2010 to 2013, due to the V6 turbo hybrid unit's lack of performance.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Renault are still mulling their future in the sport, and a possible takeover of the Lotus team, with a decision expected imminently.
However Red Bull's contract with them was not due to expire until the end of 2016 and is understood to remain in place despite discord between the two parties.
Mercedes have already refused to supply Red Bull, Ferrari are willing only to make available year-old units to sister team Toro Rosso and McLaren have a veto over Honda supplying anyone else next season.
Apart from those three, Renault are the only remaining option.
Red Bull announced some weeks ago that they had entered next year's championship and have recently been announcing a string of sponsorship renewals.
Luxury watch brand Tag Heuer has also said it is leaving McLaren for Red Bull.
"It’s an engine that will hopefully improve during the course of the year, so it’s going to be a tough start to the season for us but we’re confident we’ll make strides," said Horner.
Red Bull have Australian Daniel Ricciardo and Russian Daniil Kvyat as their two drivers.
(Reporting by Alan Baldwin, editing by Patrick Johnston)
