What is the diminutive of sow?

there is no true diminutive for sow, hog, etc, as these terms are used for adult animals. i.e. those capable of reproduction.

The name for a non-mature pig is piglet, so one could have a sow piglet.

btw the diminutive for a fountain pen is inkling.

It may well be that is only for humans that we have gendered names for the juveniles.

girl/boy, lass/lad