Which actor's real name is Walter Matasschansayasky?

Nobody's. The "urban legend" that this is the real name of Walter Matthau is false.

https://en.wikipedia.org/wiki/Walter_Matthau

Similar surnames, Matuschanskayasky, and sometimes Matashansky or Matansky have been attributed to Walter Matthau though he's always claimed that they were never his birth names. Rumours started when he took an unpaid role as adrunk in a movie and used the sobriquet instead of his real name.

https://en.wikipedia.org/wiki/Walter_Matthau

Birth Name
Walter John Matthow

Reports are that he made up "Matuschanskayasky" as a joke and that his real "real" name is Matthow. The existence of Michelle Matthow would seem to confirm this...

According to his son, Charlie Matthau, on "Larry King Live" (1985), July 14, 2000, his real name was Walter Matthow, and he changed it to be more exotic. The name Walter Matuschanskayasky was one he made up to retaliate for being "tricked" into appearing in the movie Earthquake (1974) with a much larger part than he expected.

http://www.imdb.com/name/nm0000527/bio