They could also always do the goofy ass Platybelodon.
Serperior and Decidueye aren't based on extinct species.
???Ooh! The Megalodon is a good one. I was going to say Parasaurolophus & Triceratops due to those being childhood favorites, but now that Triceratops' classification is in dispute... Maybe a parareptile? Although some argue the Venusaur line counts to keep the "all grass-type starters are based on an extinct species" thing up.
Serperior and Decidueye aren't based on extinct species.