What monster?

Tier 3 PCs are in a swamp in the Shadowfell. They are approached by an old lady riding a cow, accompanied by a (notably large) swineherd and a herd of swine.

They are, naturally, not what they appear (under the effects of seeming). The old lady is (of course) a hag, and the cow is a (modified) catoblepas, but I'm unsure what exactly I want the swineherd and swine to be. Suggestions for monsters from lore? 

Both the swineherd and swine could really be anything depending on what the theme with the Hag is. It could be a Clay Golem, Barlgura or even an Oni working for her, any would be a nasty surprise to fight. As for the swine, something quadruple would work best, perhaps a Swarm of Dretches or a pack of Wereboars?

Maybe the swine are actually people. Maybe she captures and enslaves people (of various player character species), and leads them around on leashes to be her servants (and human shields). So if the party attacks her, they're bound to hit a swine or two in the fog of war, and as it falls down dead, it reverts back to its original species form. Suddenly the party are faced with having to deal with the hag but without causing any unnecessary collateral damage. And they have to deal with the reality that they caused the death of an innocent person. 

That is... assuming the party are thoughtful people and not just a bunch of murderhobos.

But props to you for using the legendary catoblepas! That's one of my favorites! Total classic!