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