Can you transfer Ash Greninja to ultra sun from demo?
You can not transfer it from the Sun and Moon Demo. So your only option is Pokemon bank.
Can you still get Ash Greninja 2021?
Once you’ve finished Professor Kukui’s instructions and taken out the competition, Ash-Greninja will remain in your possession. Simply speak to Professor Kukui in the demo, and he’ll inquire as to what you’d like to do with the Pokemon and items you’ve obtained on your adventure.