Is Capricopian legal in Standard, Modern, and Commander?

Capricopian is legal in: Legacy, Vintage, Commander.

Capricopian

Q: What type of card is Capricopian?

Capricopian is a Creature — Goat Hydra. Its oracle text reads: "This creature enters with X +1/+1 counters on it. {2}: Put a +1/+1 counter on this creature, then you may reselect which player this creature is attacking. Only the player this creature is attacking may activate this ability and only during the declare attackers step. (It can't attack its controller.) Power/Toughness: 0/0"

Creature — Goat Hydra

This creature enters with X +1/+1 counters on it.

: Put a +1/+1 counter on this creature, then you may reselect which player this creature is attacking. Only the player this creature is attacking may activate this ability and only during the declare attackers step. (It can't attack its controller.)