The most common and most likely answer to this crossword clue is the 8 letter word MASTODON
M
A
S
T
O
D
O
N
More clues leading to the result MASTODON
- Short service to put on for old beast
- Big beast gets post keeping little old fellow under
- During Christmas, to donate, give an extinct mammal
- No points awarded to amateur coming back for old elephant
- Ring prof who’s after forest food for ancient herbivore
- Dot moans about old creature
- Head of oldest and most outlandish prehistoric beast
- Emma’s to do nowt to obstruct huge beast
- Elephant-like creature creating small commotion, trapped by guy
- Dispatched nomads to find old hoofed tusker
Buddy explains!

The straight part
We know that "old animal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old animal = mastodon.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 MASTODON mean?
MASTODON noun- Extinct elephant-like mammal of the genus Mammut that flourished worldwide from Miocene through Pleistocene times; differs from elephants and mammoths in the form of the molar teeth.