There's a varied selection of unusual creatures in this gallery. The monsters all have a cartoonish whimsical vibe about them but many are still disturbing to look at.
Slack has also made a comic called "Land of O" which can be purchased from his site. The entire book is drawn in graphite and has some incredibly detailed work in it. The content of the book matches up with some of his stranger work on display at his website.
Here is a link to one of his pencil drawings of a strange looking insect.
Here is a painting he made based on the videogame "Centipede".