To find this mob in-game, use BACKSPACE to open the World Search window, then enter Horned Melon Field in the Search field and click "Search". Remember, the field is case-sensitive and must be entered exactly as shown.
When you use a Full Water Bucket to water the field, it will display the Work In Progress icon to indicate it is not thirsty. You will need to wait until this icon expires before you can water it again.