Figured this out in singleplayer while in creative: To get one, you have to find another Rhynio, kill it for it's Pheromone, find the female, get it down to 10% health, and then feed the Pheromone to a creature you want to sacrifice, I tested this on a Trike and it worked. It only seems to do the impregnate move when it flies up to start shooting it's projectiles at you. This was done with a player spawned Rhynio rather than a map spawned. Your impregnated dino will then get "Cravings" which act like baby imprints. So far I've gotten feeding the trike kibbles, it seems like you can use a kibble above to also satiate it(asked for superior, accepted exceptional, have no idea if this affects the quality of the baby). Also has "Host Size Quality" and "Egg Quality" on the impregnated creature, however I don't know how these affect it at the moment. Discovered all of this on my own by using clues from the description of the Rhynio Pheromone and remembering how Reaper Queens work.