So this was a slightly wrong error message, what was happening was:
1. Trying to load the image hash for this pet, by looking them up at
https://pets.neopets.com/cpn/PET_NAME/1/1.png and seeing what URL it
redirects to.
2. But pets.neopets.com was rejecting our User-Agent string, which
would've been just "Ruby", since we hadn't set it otherwise. I guess
that's an explicitly banned string?
I also found that the kind of more-helpful User-Agent string I like to
write was being rejected, and I could only get it to accept something
very simple? So that's what we're using now, I guess!!