Weevil

At the end of the Sindain dungeon, you will come across a small swamp. You will need 2 things here: Sundrops and the Lafrescia Seed.

You will need to complete some of the Georama conditions in Sindain first though. First, collect enough Geostones (lava-looking rocks found in dungeon levels) and bring them back to Sindain. Then plant the Jurak trees (nose and 2 eyes) and lay down some river tiles (15-20) so that Elder Jurak will be restored in the future (might also need a few more normal trees). Talk to Elder Jurak in the future to receive the Lafrescia Seed.

Next, you need to obtain Monica's 1st monster badge. I think you obtain this by meeting the Georama conditions (listed in the menu as you collect more Geostones) for the Tailor shop to appear in the future. This involves migrating some people into Sindain and putting them in houses they like.

Once you receive the badge, Monica can transform into a Himarra (flower monster). Talk to one of these monsters (located in 1st, 3rd, and 5th levels of dungeon) to receive Sundrops.

Then go back to the small swamp, plant the Seed and use the Sundrops on it.

Of course, if you were talking about the giant swamp with King Mardan in it, then you need to use the bait given by the Firbit, and the Fishing Pole, and wait till you catch something.