The massive Doric arch was the entrance to the original 1837 Euston Station which was on Drummond Street (which used to run further east than it does now) quite close to what is now Eversholt Street. Stanford's 1870ish map shows it. It was demolished in 1961 as part of the redevelopment of the station. Some of the stones from the arch were dumped in the River Lea and some of these were retrieved in 1994 and displayed in Euston Square in 2015.
The picture source provides all the history, with pictures. And more at Cabinet Room.