slime beast

In the Servant Of Evil episode it is referred only to as "the creature" by a lizard man this monster intercepts prisoners who have escaped from the Prison of Agony, performing the dangerous task of running along the giant chains that suspend the prison.

In the script it was originally a serpent.

In the P.R.E.S.T.O. Spells Disaster episode a slime beast is kept in a vivarium as a pet by a giant who has named it Willy.