Following a tease last week, Niantic has started to release Pokemon from Generation 4 (originally discovered in the Sinnoh region) in Pokemon Go.
In a post on Pokemon Go’s official site, Niantic announced that Sinnoh Pokemon (originally found in Pokemon Diamond, Pearl, and Platinum) will be released in waves, with the first batch arriving today followed by additional Pokemon in the weeks to come. As expected, Sinnoh Pokemon are available to catch in the wild, battle in raids, or hatch in eggs and IGN has already been able to catch some ourselves.
Source: IGN.com Pokemon from Diamond and Pearl Now Available in Pokemon Go