Maybe one of the slug moth caterpillars? I can't find that one but this looks a bit like it.

Here is a link that might be useful:

Yep, one of the Crowned Slug caterpillars. All of the caterpillars in this little group have the capability of stinging when you touch them, some painfully so. Don't touch!

Have you found a lot of them?

Yes, I have found a lot. Why have I not seen them before? Are they good or bad for the garden?
gmom

I would consider them bad because they are going to eat your plants but personally I wouldn't kill them. I'd use something (not your bare hand) to transfer them to a plant they'll eat that you don't care about. I don't use pesticides and rely on beneficial insects for control but I have no idea what preys on slug moth cats. Then again I grow plants just for the caterpillars to eat.

Probably not a good idea to move caterpillars from plant to plant if your intention is to save them. I think that this group of caterpillars can be found on several different kinds of host plants but they don't like being transferred from a different kind than they hatched on. Does that make sense?

Your decision to get rid of this caterpillar should be based solely on how many you've seen on your potatoes and how much damage that they're doing. Got a bunch of them? Put a glove on and put them into a container of soapy water, alcohol, or other. Then dispose. Birds won't eat them.

If you only have one or two, share and share alike is a good policy.

Another reason to get rid of as many as you can is to avoid the possibility of an accidental sting. The slug caterpillars have a bad reputation for painful stings...many of us have personal experiece with that.

Why haven't you seen them before? These would be pretty easy to miss, I think. I've never seen one in person and have never heard of them being found in large numbers.

Thanks for posting such a good image.