Still one that is found in Himalayan mythology (4)
I believe the answer is:
yeti
'that is found in himalayan mythology' is the definition.
(I've seen this before)
'still one' is the wordplay.
'still' becomes 'yet' ('yet' can be a synonym of 'still').
'one' becomes 'i' (Roman numeral).
'yet'+'i'='YETI'
(Other definitions for yeti that I've seen before include "creature of dubious existence" , "An abominable animal!" , "fictional creature" , "The Abominable Snowman in Tibetan" , "Himalayan mystery" .)