[rob74]

Yes, the form really does ask if a person is bringing in snails.

Even more curiously, it asks for animals in general, and then specifically for snails. I wonder what it is about snails specifically that US Customs are/were so interested in?

[caseyohara]

Biosecurity/agrosecurity. Some snails are highly invasive and destructive to crops so they are banned or tightly regulated by the USDA.

[jlarocco]

I'm not sure what they were looking for, specifically, but snails can be bad news: https://bouldercolorado.gov/services/new-zealand-mudsnails