It's a snek. Beware of teh snek.
It appeared in in Kinda and Snakedance - two GOAT Peter Based-ison stories - and returned for an audio that I haven't heard.
It's the most evil creature in all of existence ever... y'know, just like the Master, the Daleks, Sutekh, the Beast, the Black Guardian, Fenric, Kronos, the Great Vampires, whatever the fuck the Hybrid was supposed to be...
It took great pleasure from sliding deep inside Tegan after having roofied her.
Best ship in Who.
Pretty spooky, one of Classic Who's best monsters, right up there with the Mandrels.