Fungusaur was a staple of the early core editions (nostalgia).
When Blake announced the upcoming Companion App, he referenced the fact that Fungusaur has gained the Dinosaur subtype, possibly hinting at a future updated printing.
It's ability is essentially an Enrage mechanic, so fits dino decks beyond simply having the Dino subtype.
The Fungus subtype is relevant with Dominaria, and the likely addition of more support in the future return to Ravnica.
Having a +1/+1 counters mechanic makes it relevant, whether it's the current merfolk or the future Simic.
Will We See a Fungusaur Reprint in M19?