Is Horned Helm legal in Standard, Modern, and Commander?

Horned Helm is legal in: Modern, Legacy, Vintage, Commander, Pauper.

What type of card is Horned Helm?

Horned Helm is an Artifact — Equipment. Its oracle text reads: "Equipped creature gets +1/+1 and has trample. {G}{G}: Attach this Equipment to target creature you control. Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery.)"