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