T-Rexes

[Zantetsuken]

Sounds like you want them as NMs Zan.

Not exactly NMs..

Unless you consider the "Guardian of the Grove" mobs NMs.

They are just mobs that occupy a remote zone, but they could drop exclusive items like 'Rex Hide' or 'Dinosaur bone' that could be used for crafting some prestige armor (ie. with stats like Darksteel Haubergeon, but with a 'Tortoiseshell Scale Mail' style design)

Perhaps add an achievement for killing them as well, but overall, make them a dangerous oddity in the world.