In another surprise, veteran back Matt Giteau on Thursday was left out of the 30-man squad announced by Australia coach Robbie Deans for the September 9-October 23 tournament.
“We all felt it was in the best interests, both of Rocky and of the team, to relieve him of the extra duties, with James’s recent experience making him the best option to take over in the role,” Deans said.
Queensland Reds lock Horwill will become Australia’s 77th Test captain when the Wallabies have their final hit-out before the World Cup in next week’s Tri-Nations Test against the All Blacks in Brisbane.
Elsom was replaced after leading the Wallabies in their last 24 internationals, but retains his place in the squad as one of the options for lock.
“We all want the same thing now so it’s all heading towards that,” Elsom told reporters.
“I think it’s important that everyone gets behind the captain, it can be a difficult job at times and you need everyone right behind you there.”
Deans, who reinforced his reputation for making big selection calls, praised Elsom for his grace in losing the captaincy.
“Rocky’s response was impressive and spontaneous and he simply said ‘whatever was in the best interests of the team’, so he remains an important person and important leader within the group,” Deans said.
“It is well documented that Rocky came into the Test season off a limited background in terms of preparation, having missed all but 60 minutes of Super Rugby due to injury.
“As the incumbent captain, he warranted the opportunity to continue in the role but it has become clear that the additional responsibility has been a distraction that Rocky doesn’t need.”
Horwill said he was surprised and honoured to be handed the Wallabies captaincy after leading the Reds to this year’s Super 15 southern hemisphere title.
“It’s very much a surprise. It’s something I feel very privileged to be given the opportunity to represent your country, and to be announced as captain is just another step and I’m greatly honoured for the opportunity,” he said.
Giteau, who has 92 caps and was the Wallabies’ highest-paid player and star playmaker until last year, missed out after hinting via his Twitter account on Wednesday that he would not be at the World Cup.
In Giteau’s place, Deans selected NSW Waratahs playmaker Berrick Barnes as the fly-half back-up to Quade Cooper and Nick Phipps of the Melbourne Rebels as a third specialist scrum-half behind Will Genia and Luke Burgess.
Elsom is one of eight Wallabies who played at the 2007 tournament in France, where Australia bowed out in the quarter-finals.
The others from 2007 who are still in the squad are centre Adam Ashley-Cooper, Barnes, winger Drew Mitchell, hooker Stephen Moore, No 8 Wycliff Palu and locks Nathan Sharpe and Dan Vickerman.
Among the notables who missed out on World Cup selection were Mark Gerrard, Lachie Turner, Matt Hodgson and Dean Mumm.