California butterfly expert/author Art Sharpiro identified it as the lovely White-lined Sphinx Moth Hyles lineata. The caterpillar of this species comes in several color morphs and has a little horn on his butt.
Art wrote, It's a fairly extreme generalist, but does not have a documented liking for iridoid-glycoside plants-- the only record I find for it on such is for Plantago. ... The moth is a long-distance seasonal migrant.
The only pictures we got were of the caterpillar, and they are pretty poor, but because this is unusual I am documenting it. To see a photo and information on this lovely moth, click on this link.
Our thumbnail photos of the caterpillar can be clicked on to show it screen-sized.
The photo below shows where the caterpillar was in the pond. Click on it to enlarge it (~3 MB)
