Haggar is a Side eastward of Jetstone. Its relationship with Jetstone has traditionally been troublesome. They originally hoped to exploit Jetstone's troubles by annexing one or more of its cities. However, Chief Warlord Ossomer succeeded in intimidating Haggar to the point where they agreed to join the Royal Crown Coalition against Gobwin Knob.
Real World References
Haggar seems to refer to the clothing company by the same name.
Prince Sammy Haggar is based on the performer Sammy Hagar.
Haggar may also refer to the comic strip (and titular character) Hagar the Horrible.
Haggar was the name of an enemy witch in the cartoon Voltron, though there are no obvious references to this so far.
Likewise, Haggar is the name of a Capcom character who was a professional wrestler and mayor, but there is no clear reference.