The most common and most likely answer to this crossword clue is the 8 letter word BULLFROG
I've seen this crossword clue in The Daily Telegraph
B
U
L
L
F
R
O
G
More clues leading to the result BULLFROG
- Creature, male, by river shrouded in mist
- Fifty points scored for unravelling head girl’s large jumper
- He may splash fully tailless right into marsh
- Amphibian, male, Rex found in the gloom
- Maybe Hereford mist shrouds river and pond dweller
- Running full, river in swampy area suits amphibian
- 12 Bovine amphibian creature
- Amphibian, male, right to hide in low cloud
Buddy explains!

The straight part
We know that "Creature" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case creature = bullfrog.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 BULLFROG mean?
BULLFROG noun- Any of various frogs having a croak that resembles the bellow of a cow or bull.