Sven is seriously injured by an attack from Haggar, and transported off to get medical attention. Yet in the end, he is seen celebrating with the others (this is due to the writers' editing out the Go-Lions original story's ending death, by placing a celebration scene with all, including him, in it from near the beginning in its place).
Lance, not Hunk, is the one riding the horses with Pidge and Allura though Hunk was the one preparing to do this while Lance was with Keith going to Sven.
Keith says "Sven is Voltron's right arm", while Sven pilots the Blue Lion, which forms the Right Leg of Voltron. However, he is metaphorically referring to his devotion and indispensability to the team, not making a mistake about the job that his friend has held since the team was founded.