The spider plays a minor role in "Marceline's Closet." When Jake tries to escape Marceline's closet by digging a small hole in the wall with a spoon, the spider chases the shrunken Jake out. The spider then pushes Marceline's lamp off her nightstand. Near the end of the episode, the spider distracts Jake when he and Finn try to break out of Marceline's house. It ends up biting Jake.
It is pale white and has sixteen silver-green eyes. It has actual teeth and even gums instead of fangs, which is possibly a mutation. It has six legs instead of eight, so it is technically an insect and not an arachnid.
The spider in "A Glitch is a Glitch" is very similar in appearance, with sixteen eyes and six legs, though it is black. A more similar colored spider can be seen in the credits of the episode, though this one has a simple pattern on its back. The similarity between the spiders' appearance may show that they are of the same mutant species.