Mermaid stickers (000013b)
You have two options.
Mermaid & friends or just mermaids!
These fit perfectly in my sticker organizers found here:
https://www.etsy.com/listing/230133621/sticker-storage
I also sell a min sampler version of this here:
https://www.etsy.com/listing/232741815/mermaid-sampler-pack