Horne was previously part of a group of Fife youngsters who achieved a rare double, playing in the Bell Baxter High School team who won the Bell Lawrie Scottish Schools Cup and then in the Howe of Fife squad who lifted the Scottish Youth League Cup.
[9] He made his debut for Glasgow Warriors as a replacement in the Celtic League match against Ulster in March 2009, his first start following the same month against Munster at Firhill, and he signed a full-time contract in the summer.
He was in the Scotland team who beat Italy in the under-19 international in February at Parabagio, near Milan, and he then played for the national under-20 squad in all five matches in the IRB Junior World Championship in Wales in June, starting in three of the games.
He scored all of Scotland's points with six penalty goals in the 18–17 win against Wales in the 2009 Six Nations Under-20 Championship match at McDiarmid Park, Perth, in February.
Peter took his under-20-year's tally to 63 points when he scored 39 in starting in all five matches in the IRB Junior World Championship in Japan in June.
Horne made his full international debut for Scotland on the 2013 summer tour to South Africa, during which he picked up a serious knee injury which kept him sidelined for almost a year.