Elite Trial Flying: How Does it Work for NPCs?

In the Indigo Disk to earn the right to battle Amarys you have to complete a flying trial. Amarys says that with some effort Miraidon or Koraidon can fly, but this can't true of all Pokemon. What if a trainer never caught a Pokemon that can fly? Would they be given a Skarmory to use temporarily.? Would they have an advantage or disadvantage over trainers that use different species?