Horse is an odd form of such natural features (9)

I believe the answer is:

mountains

I'm a little stuck... Click here to teach me more about this clue!

'form of such natural features' is the definition.
Although both the answer and definition are plural nouns, I don't understand how they can define each other.

'horse is an odd' is the wordplay.
'horse' becomes 'mount' (mount is a kind of horse).
'odd' indicates anagramming the letters.
'is'+'an'='isan'
'isan' is an anagram of 'ains'.
'mount'+'ains'='MOUNTAINS'

Can you help me to learn more?

(Other definitions for mountains that I've seen before include "Projecting land masses" , "Peaks" .)

I've seen this clue in The Irish Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.