Complex sentence: Devon Loch (1946--1963) was a famous racehorse that is probably best remembered for its involvement in the 1956 Grand National steeplechase, when owned by HM Queen Elizabeth, the Queen Mother.
Split into simpler sentences: Devon Loch (1946--1963) was a famous racehorse. Its most memorable moment by far came in the 1956 Grand National steeplechase, when owned by HM Queen Elizabeth, the Queen Mother.