Is Sentry Bot legal in Standard, Modern, and Commander?

Sentry Bot is legal in: Legacy, Vintage, Commander.

What type of card is Sentry Bot?

Sentry Bot is an Artifact Creature — Robot. Its oracle text reads: "Flash This spell costs {1} less to cast for each creature attacking you. When this creature enters, you get {E} for each creature attacking you. At the beginning of combat on your turn, you may pay {E}{E}{E}. If you do, put a +1/+1 counter on each creature you control."