Mantid Nymphs are the Mesmers of the Mantid species, found in the Charr Homelands. They are possessive of a unique "self-resurrection" ability, that sometimes allows them to get up again after seemingly dying, mirroring some species of real-world insects. The ability is quite limited, however, so the Nymphs can be killed once and for all without too much trouble.
Mantids in Eye of the North have the innate ability to randomly come back to life after dying. This is not a bug, according to Andrew McLeod, Arenanet's creature designer, and is "intended to make them feel more like big insects- you stomp on them, but they still get up and crawl around". Frozen Soil will not prevent this, and Mantis revived in such a manner will not offer points towards your title tracks. However, the second death of such a creature will still leave an expoitable corpse.
"Nymph" is the scientific term used to define an invertebrate in the immature stage of development.