Very hard to tell by the picture but do a google search on- Pine sawfly larvae Pickleworm larvae

I'm tempted to call it an Abbott's Sphinx caterpillar. It is certainly one of the Sphingidae, this species without the horn, but a false eyespot. Would be most helpful to know what plant you found it on and in what part of the world.

My guess is it normally does not look like that. Probably changed color while it searches for a place to pupate. Its head looks distinctive. I suggest taking your best head shot and cropping it down so we can get a good look at its face. Might help ID it.

Iris, what kind of plant are those cats ravaging? The Mexican Sweet Herb is very pretty. I guess you could call it a frogfruit, because it's so closely related. It looks like it would make a nice front of the border, or container plant. Do you get many butterflies on the flowers? Do uou know what species of them are the best as host plants. One side of my house is shaded by Oak, and Cottonwood. Grass won't grow there so I have ferns and Hostas, and there's a few shade loving Carex sedge in there too. I noticed a bunch of little plantains in there, that are tiny because of the dry, hot, weather. I'm going to start watering and fertilizing them for the Buckeyes. When I went to harvest a syriaca stalk tonight, I found yet another Monarch cat on it.