Is Riders of Rohan legal in Standard, Modern, and Commander?

Riders of Rohan is legal in: Legacy, Vintage, Commander.

What type of card is Riders of Rohan?

Riders of Rohan is a Creature — Human Knight. Its oracle text reads: "When this creature enters, create two 2/2 red Human Knight creature tokens with trample and haste. Dash {4}{R}{W} (You may cast this spell for its dash cost. If you do, it gains haste, and it's returned from the battlefield to its owner's hand at the beginning of the next end step.)"

Riders of Rohan

Creature — Human Knight

When this creature enters, create two 2/2 red Human Knight creature tokens with trample and haste.

Dash (You may cast this spell for its dash cost. If you do, it gains haste, and it's returned from the battlefield to its owner's hand at the beginning of the next end step.)