A Baby Dinosaur hatches from a mysterious egg that can be found, rarely, in a raptor nest location. A baby dinosaur eventually grows into a raptor after ~50 days. You will then be given the choice whether to accept the grown raptor into your trek.
Warning: If you choose not to let a newly-grown raptor join your trek, it will be placed on the map as a hostile animal and may get aggroed if you remain near it.