Here is the full list of all 9 Skeleton Rider trophies - there are 3 bronze, 5 silver and 1 gold.
Complete the game.
Complete fourth cup.
Complete third cup.
Complete first track in 'Rocks' cup without falling.
Finish second cup.
Hit yor head 50 times.
100 Restarts.
Finish first cup.
Finish first two levels.