Ben firsts transforms into Spidermonkey when he has to fight off his Anodite grandmother in the episode "What Are Little Girls Made Of?".
Abilities
Spidermonkey is an alien with 4 arms, 4 eyes and 2 legs, a feature Spidermonkey has in common with Fourarms and Wildmutt because he is beastly. Spidermonkey can spin webs from his tail (seemingly inspired by Stinkfly's ability to eject sticky goop), has supernatural adhesive skills, and has superhuman agility. Also, he surprisingly can swim.
Weaknesses
Although incredibly agile it can get tangled by its own web.
Origins
Spidermonkey is an Arachnichimp from the planet Aranhaschimmia. The name "Arachnichimp" is derived from arachnid, the name of a group of animals including spiders, scorpions, and ticks, among others, and chimpanzee, a type of ape. Oddly, Spidermonkey has a tail, a feature chimps (and indeed, the hominoids as a whole), do not posses.