Gastropod

The Sectoms, (PROSE: The Twin Dilemma) better known simply as Gastropods, were a race of humanoid slugs that devoured the vegetation of the planet Jaconda, led by Mestor the Magnificent.

Biology
The Gastropods resembled humanoid slugs. They left a behind them when they moved, and laid eggs which could remain fertile in space. Apparently only the leader, Mestor, could communicate verbally; the rest obeyed his commands without question. (TV: The Twin Dilemma)

Abilities
Gastropods had a form of mental powers which they could use to either kill individuals or take over their mind and communicate through them. They could also communicate telepathically. Their leader Mestor managed to open the doors of the Doctor's TARDIS through its psychic power. (TV: The Twin Dilemma)

History
According to the Jacondans' legends, a Jacondan queen offended their sun god, who punished her by forcing her to give birth to a Gastropod. The Gastropod had countless children who devoured all of the plants, leaving the Jacondans on the verge of starvation before the sun god sent a drought which wiped them out.

The Sixth Doctor, believing that there was "more than a grain of truth" in the story, determined that the Gastropods somehow got to the planet and died out, leaving behind clutches of eggs which lay dormant for centuries. (PROSE: The Twin Dilemma)

Jaconda became a despoiled wasteland once more when the eggs somehow hatched, the Gastropods making the Jacondans their slaves. In 2310, their leader, Mestor, planned to make the Jaconda's sun go supernova so as to scatter Gastropod eggs into space. The Sixth Doctor killed Mestor and ended the reign of the Gastropods. (TV: The Twin Dilemma)

Behind the scenes

 * According to The Brilliant Book 2012, a book that contains non-narrative-based information, there was a fossilised Gastropod in Dr Henry Fizzog's Museum of the Macabre on Sardicktown.
 * The name for this monster obviously derives from s, the biological class of slugs and snails.