Is Knighted Myr legal in Standard, Modern, and Commander?

Knighted Myr is legal in: Modern, Legacy, Vintage, Commander, Pauper.

Knighted Myr

Q: What type of card is Knighted Myr?

Knighted Myr is an Artifact Creature — Myr Knight. Its oracle text reads: "{2}{W}: Adapt 1. (If this creature has no +1/+1 counters on it, put a +1/+1 counter on it.) Whenever one or more +1/+1 counters are put on this creature, it gains double strike until end of turn."