An X-Wing with one extra place for the number, a fin, in a box beside a corner. The number still leaves the X-Wing's column, but only inside the fin's box.
123456789
123456789
5
9
6
8
5
2
4
5
8
7
3
7
1
2
5
9
8
7
9
6
5
2
8
4
5
5
1
5
6
5
5
1
The X-Wing's cornersThe finLoses a pencil mark
Why only the fin's box
A square can only be cleared if it also sees the fin, because the fin might be the 5. In column 1, those squares are inside the fin's box.
How to spot one
Look for X-Wings that fail by one square. If the stray place shares a box with a corner, you have a finned X-Wing.
What it gives you
Usually one removal, sometimes two. Finned fish turn up on tough boards.
Practise Finned X-Wing
Boards that need it, with the owl's hints when you're stuck.