This is a good question, and I don’t know why people are being jerks about it.
Just checking the OED. “Sow” for “pig” goes back to Indo-European. “Sow” for “bear” is only attested in the OED from 1976. “Boar” for “pig” goes back to common Germanic; “boar” for “bear” is not mentioned at all as a specific term, but the forms are similar and it looks like they’re occasionally confused from the Middle English period. “Sloth,” on the other hand, is more traditional: it goes back to 1452.
So, as an educated guess, I would say that once the “boar” was applied to “bear” in error, “sow” naturally followed to go with it by analogy.
The bull / cow / calf thing is a more straightforward analogy, applied to many other very large animals.