• 3 poses total
• in game The Sims 4 Poses
• place two teleporters in the same spot to pose
  (mermaid sim needs to go swimming first, so the tail appears and then use the teleporter.
  alternatively, you can use a CC tail, like this one OR use a cheat to get the mermaid tail on land:
  traits.equip_trait trait_OccultMermaid_MermaidForm )

you’ll need:

• pose player
• teleport any sim