10 Doctor Who Villains You're Surprised Only Appeared Once
1. The Krynoid
'The Seeds of Doom' is an absolute cracker of a Tom Baker story. It begins by riffing on 'The Thing From Another World' long before John Carpenter and Kurt Russell did to great acclaim in 1982. After the Doctor and Sarah's Antarctic sojourn, they return to the UK to stop unhinged millionaire Harrison Chase from playing in his green cathedral and unleashing the terrifying Krynoid upon the world.
The Krynoid are a great creation by Zygon creator Robert Banks Stewart, carniverous plant life that consume animal life. Their pods can transform other beings into plant life too, prompting the Doctor to recall that on planets where the Krynoid take root, all animal life ceased to exist. The Krynoid have dug their tendrils into various bits of Doctor Who spin-off media, but haven't yet returned to the series.
As plant-based diets have become increasingly popular in recent years, and with the return of Russell T Davies to the show the time is surely right for a Krynoid return. It's in RTD's wheelhouse to use an old Doctor Who monster to comment on our contemporary society, and a story about plants that eat humans feels relevant at a time where we're all questioning our impact on nature.