Goo

These are somewhat simpler creatures, they are all digestive juices and perfect for unconventional vore and just engulfing other things in their way. Some Goo's or Jelly's require only a touch to begin engulfing their prey, though the rare few keep enough surface tension that a break in the 'skin' must first be created. Typically when consumed by a slime, the prey will be digested into more slime, though it's not unheard of that the prey itself be ejected as another slime-being. 

You really only have to decide on how firm your goo is, weither its drippy or not, and mainly what color and attitude it has. Though simple in structure, they can become slightly more complex as shapeshifting or even the ability to hold the shape of another creature come into play. Though they may seem difficult to use as prey, depending on firmness they can be sucked up or swallowed much like any other creature. The real trouble is digesting them. Since many slimes are made up of the aforementioned digestive juices, they may be able to move freely within the body once ingested, making for some rather interesting scenarios. Make them smaller, and they become the perfect Jello shots. Though technically interchangeable, jelly more often refers to a creature with a semi-solid form than slime or goo. It should also be noted that a slime/goo/jelly-creature (i.e. slime-girl) refers to a slime that bears the semblance and form of the creature while retaining most characteristics of a slime.