That’s a good thought, but…
And so on…
Although you are correct that unreleased pets have a Weight of zero, ALL currently released pets have a Weight of 10. That adds up very quickly to well past 100%. It could simply be a matter all of the pets being on the drop table (as they are in the game files), with code that says:
Roll a pet for a rescue
Check that pet’s “Drop Chance Weight”.
If that pet’s “Drop Chance Weight” = 0, then reroll pet and check again, else spawn that pet’s rescue.
All the devs would need to do on a pet’s launch is simply change that pet’s Drop Chance Weight to a non-zero number to make the pet appear in pet rescues.
Yeah, that’s likely what’s happening here. The devs can simply “toggle” a pet to be spawnable in a rescue by changing its Drop Chance Weight to a non-zero value, and vice versa.