To get a specific Riftborn pet, do you have to toss in that species of pet? E.g. if I want a Riftborn Scootle, do I have to toss in a Scootle, or is it randomized?

You'd have to toss in a Scootle. If the pet you throw in doesn't currently have a Riftborn color available, then it'll randomly turn into an RA-001 or RA-002

Thanks so much ! :)