Megawhaitsia, a large therocephalian synapsid from the Latest Permian in East Russia. Known only by a maxilla, with a skull estimated to be Artwork by licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works License.
Sinomegaceros pachyosteus by ~RomanYevseyev Deer can be surprisingly dangerous as it is. I'm tempted to have the PCs-- next time they run out of food-- pass near some does that are large but not otherwise unusual. And then have the stag bust out.