The London Youma

The so-called "London Youma" was a youma from the Dark Kingdom who appeared in episode 042. He appeared on briefly in Minako's flashback of her time in London. Katarina was chasing a male criminal who took a child hostage and then turned into the "London Youma." Sailor V arrived on the scene and attacked, managing to both destroy the youma and save the little girl.
The "London Youma" was never actually named in the series. He is called the "London Youma" by fans only to refer to him; it is not his true name. He was the only youma ever to attack somewhere besides Japan.

Related anime:

Sailor Moon TV, 1992