For a monster girl, such as a mermaid or lamia whose legs are fused into a tail, but only starting somewhere below the waist, leaving the thighs still separated at the top.
At minimum, there must be enough room to run a string from front to back through the middle. Having an ass or vagina alone isn't enough.
This is frequently part of an artist's attempt to solve the so-called "mermaid problem."
See also
Forked Tail : One tail that splits into two around midway.
