Pentobarbital is what vets use to euthanize pets. It's manufactered in the US and is quite common though there are only two US companies that manufacture in - one in NY and one in CA. It's also used sometimes in lower doses to control seizures in humans.
Pentobarbital is typically not what is used in the US when putting someone to death by lethal injection. More typically a cocktail of drugs is used: sodium thiopental, followed by pancuronium bromide (a muscle relaxant) and potassium chloride which is what stops the heart. Pentobarbital has been used in lethal injections on humans though. Each state seems to have their prefered drug or cocktail of drugs of choice which may have a lot to do with availability and particularly pricing.
Pentobarbital is rarely used in humans as an anesthetic except in some experimental cases. There are sooooo many preferable anesthetic drugs to use instead.
