The Spiny Oak Slug, Euclea delphinii, is not a slug at all, but a flamboyantly decorated caterpillar, looking scared of its own shadow:
Can you see that it is wide and flat, not cylindrical like most caterpillars? Rows of yellow or orange stinging spines line the caterpillar from head to rear, with longer pairs at the head and the tail. They have sharp black tips, shown in close-up below.
These venomous spines deter predators, and touching them is a mistake! They can be extremely painful. If you get a spine embedded in your skin, try to remove it with Scotchtape, and then use baking soda. Some people may even have an allergic reaction that needs medical attention. (Information from insectidentification.org)
My caterpillar was stationary, but slug caterpillars are so-called because they move rather like slugs. They lack any functional prolegs. Instead, they have suckers, and produce a sort of liquid silk lubricant, so they move by undulating across the smooth leaf surface. The Caterpillar Lab has a great video of one in motion:
And a final pose: