No, a Keydet is not a Kangaroo, although the term "Keydets" has been used over the years to refer to VMI's athletic teams and the kangaroo has become the established mascot.
The story behind the kangaroo is relatively simple. Back in 1947, two VMI cheerleaders saw a picture of a kangaroo on the front of the magazine and realized how uncommon the animal was as a mascot. As one of the mascots was finally procured, a contest was held to give the creature an appropriate name. The prize winning name was "TD Bound."
Sometime later the kangaroo's name was changed to "Moe" in order that he might be associated in all sports at VMI and not just football. So far in history there have been four real "Moes", the last of which was actually a wallaby, the smaller relative of the kangaroo. A kangaroo costume worn by a VMI cheerleader then replaced the live mascot in the 70's so there could be a mascot representative at any VMI contest at home or on the road.
The source and/or meaning of the nickname "Keydets" is not as easy to find. The problem lies in the fact that there is no definite meaning that has been found for the word. There have been plenty of explanations offered, but none have been convincingly substantiated. The United States Military Academy claims that it was a word used to denote the gray of the standard uniform of a cadet. One less factual definition is that due to the Southern drawl of some of the members of the VMI Corps, the common term cadet was transformed into "Keydet".
VMI has had many nicknames through the years. Probably the longest standing name used to refer to its team, especially in football, was the "Flying Squadron." The nickname got started back in 1917 at a football game between VMI and North Carolina. The name stuck and became more popular during the 1920 football season. It wasn't until the 30's that the term "Keydet" came into use in conjunction with the athletic teams.
Regardless of which definition is correct, the word "Keydet" like kangaroo, will continue to identify VMI's athletic teams.