|Scientific name||Mesonyx obtusidens|
|Home era||Mid-Late Eocene|
|Appearances||Primeval: Lost in Time|
Mesonyx was a large, hooved, wolf-like mammal from the Eocene Epoch with large, tusk-like fangs on its lower jaw.
A pair of Mesonyx come through an anomaly to Springfield High School in 2002. While one Mesonyx goes to the school's cafeteria, looking for food, the other prowls through the school corridors. The latter Mesonyx eventually sees Walter and Sean break into the school, looking for Walter's missing cellphone, and stalks and follows them to the boiler room, where the anomaly is.
While Sean and Walter are distracted, staring at the anomaly, the Mesonyx attacks them from behind, almost killing Walter. However, the two kids notice the creature and beat it with a pole in self-defence. Wounded, the Mesonyx calls for help, attracting the other Mesonyx to the boiler room, before retreating back through the anomaly.
The remaining Mesonyx then comes into the boiler room and attacks Sean and Walter, killing Walter and biting his head off. As the Mesonyx then turns its attention to Sean, the horrified boy falls backwards, through the anomaly. The Mesonyx tries to follow Sean back through the closing anomaly, but when the creature is halfway through, the anomaly closes on it, cutting it in half and killing it, and leaving one half of its body in the Eocene, the other in 2002.