Yes, it comes up in Himalayan mythology (5)
I believe the answer is:
yetis
'yes it comes up in himalayan mythology' is the definition.
I can't judge whether this defines the answer.
'yes it comes up in' is the wordplay.
'comes up' is a reversal indicator (in a down clue, letters go up).
'in' means one lot of letters goes inside another.
'it' in reverse letter order is 'ti'.
'yes' going around 'ti' is 'YETIS'.
Can you help me to learn more?
(Other definitions for yetis that I've seen before include "Mythical creatures" , "Rarely seen group of creatures" , "fantastic beasts" , "More than one mythic creature" , "Mountain monsters" .)