2004: Yamaha DT125X launched at the end of the year, a few months after the birth of the RE motorcycle. No changes since.
Yamaha DT125RE: The current, off-road biased version with very similar spec to the X. Differences include a higher seat, traillie-style wheels and knobbly tyres, smaller brake discs and price (it’s marginally cheaper).
Yamaha DT125R: Introduced in 1988 (but the engine had been around long before then), it launched a million new riders in to motorcycling. Current versions are updates of the R model but much, including the engine, is the same. Discontinued in 2004.