Rhyniognatha Pheromone is an item used to lure a female Rhyniognatha so it can lay an egg in the creature that consumed it. To get a pheromone it is necessary to find a male Rhyniognatha and kill him (the female does not drop). This can be done with the help of a tamed creature or a ranged weapon, but be careful not to be paralyzed in resin.
After acquiring the pheromone, use it on one of your "big" tamed creatures to attract a female Rhyniognatha, the creature must have at least 300 drag weight. Pheromone attract effect lasts 300s (5min)