Gonzo History are running a monster design competition [http://gonzohistorygaming.blogspot.hk/2017/11/the-first-monster-man-contest.html]. The core of the rules is to find yourself a cheap toy and then write it up as a monster for the role playing game of your choice. Here’s my entry, using a toy I got from a capsule machine.
Shell Witches are merfolk who’ve been banished from the Free Sea Nations for practicing forbidden magics. They may not enter open water and so they end up in shallow bays, dependent on their landsuits to protect them from drying out when hunting for food on land. They are hostile and aggressive to all land dwelling sentients. Though not all Shell Witches attack on sight, many will and some even set ambushes. They have no qualms killing a traveller for their supplies. Their dwellings are underwater caves that the Shell Witch will have carved with water magics. Inside are collections of stolen magical items and food stores. It is exceptionally rare to encounter more than one Shell Witch at a time as they are not social.
The landsuit is only lightly armoured but provides additional protection if wet. In combat a Shell Witch will use their mobility to stay behind cover whilst using water magics to attack from range. They are unwilling to engage in melee and will adopt skirmish tactics if pressured. They carry a shell carved with runes which acts as the focus for their magic.