Horsman qualified to represent Wales in 2005, having completed his three-year residency period, after rejecting a call from England to tour Canada in 2005. He gained representative honours for England at U18, U19 and U21 levels.

Horsman had spells at Bath and the Celtic Warriors, before moving to the Worcester Warriors where he has become recognised as one of the Guinness Premiership’s most destructive tight heads. Horsman signed a contract extension with the club to keep him at Sixways until 2010.

Horsman missed the 2006 RBS Six Nations due to an ankle injury but was included in the Wales squad for the Invesco Perpetual Series in the autumn of the same year, appearing in the victory over the Pacific Islands. He also starred in the 2007 RBS Six Nations against Ireland, France, Italy and England; facing the country of his birth for the first time in a Welsh jersey the prop scored a 14th minute try in the victory.

In April 2007 Horsman was due to be rested from the squad set to depart for Australia for the two-Test tour. However, due to injuries within the camp, Horsman flew out to Brisbane to provide bench cover for the second Test. He made a substitute appearance in the second Test defeat and in June was named in Wales's 41-man preliminary Rugby World Cup 2007 summer training squad.

After the Test defeat to England Horsman was named in Jenkins's final 30 man RWC squad and later that month played in the Invesco Perpetual Summer Series match against World Cup host nation France. He made two appearances for Wales in the Rugby World Cup, making the starting line-up against Japan at the Millennium Stadium and Fiji in Nantes.