Until you complete the Sinnoh PokéDex, the Pokémon in the Trophy Garden will not change. However, once you obtain the National PokéDex, Pokémon from other regions will appear. Bulbapedia says:
After the player has gotten the National Pokédex upgrade, however, Pokémon that do not otherwise appear in Sinnoh will be placed in the Trophy Garden by Backlot's butler to cover up for his fibs, with a different one there daily. The counter is not reset at midnight, but after 24 hours. It is possible to have Backlot mention a different Pokémon, simply by soft-resetting. The last two Pokémon placed in the garden at any given time will remain available until more Pokémon are placed in the garden.
Because you can soft-reset for which Pokémon Backlot places in his garden, it's safe to say that the special Pokémon in the garden is randomly generated upon speaking with Mr. Backlot.