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.
Finish second cup.
Complete first track in 'Rocks' cup without falling.
Finish first cup.
Hit yor head 50 times.
100 Restarts.
Finish first two levels.