This is the 13268th most frequent Russian word.
монголом
"Монголом" is the instrumental case of "монгол" in Russian, meaning "Mongol" or "a Mongolian person."
The word 'монголом' is used as a predicate in the instrumental case to denote someone's identity or ethnicity.
Я встречал его в музее, он говорил, что был монголом.
I met him at the museum; he said he was a Mongol.
Here, 'монголом' is also in the instrumental case to express a temporary role or state someone adopts.
Чтобы понять культуру, он решил стать монголом на время экспедиции.
To understand the culture, he decided to become a Mongol during the expedition.
In this sentence, 'монголом' identifies the origin of inspiration, again in the instrumental case to specify the source or influence.