The most common and most likely answer to this crossword clue is the 4 letter word YETI
Y
E
T
I
More clues leading to the result YETI
- Legendary creature still seen to west of island
- There’s still one monster
- Still, one that’s never been authenticated
- Mountain creature not seen — one that’s still on top?
- Legendary Highlander: one’s got so far ahead
- Legendary creature, until now, first in index
- Himalayan legend still current?
- Even I find this creature fantastic
- Despite everything one finds legendary creature
- However, one is a legendary monster
Buddy explains!

The straight part
We know that "Mountain creature not seen" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mountain creature not seen = yeti.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does YETI mean?
YETI noun- An unidentified humanoid animal said to live in the Himalayas