8 value, zero cost Sidekick, this dude cannot be moved by opposing card effects... Hang on, let me read that again...
Yep. This card is good. Really good.
Eights are a popular value, able to be used by many decks. However prior to the release of Pedro, for the majority of decks the only real option on hearts was the Quickdraw Handgun (once Paralysis Mark received its much-needed errata, anyway). While the Handgun is by no means a bad card, if you were stacking the value then bringing four of them could result in them clogging up your play hand, especially if you were also using other Weapons or costly goods in your deck. Pedro's zero cost means that he's never going to get stuck in your hand as long as you've got a dude to attach him to. While a dude can only have one Horse, if there's already a Pedro in play you can just attach a new one and discard the old one to cycle the card from your hand.
Sidekicks are useful to have for decks that are looking to get involved in shootouts. If you take your casualties on them instead of your dudes, you've lessened the impact of a lost or tied shootout round, while your opponent may have lost a far more valuable dude. This capacity to absorb attrition is great for long, drawn out shootouts, or those times when a dude is forced to fight on their own. As their purpose is to absorb casualties and cycle back into the deck, they also help prevent excessive degeneration of your deck. Usually, you'd pay two ghost rock for a Sidekick, but not here.
The trait preventing your dude from being moved by opposing card effects is extremely useful. Once they are riding their trusty mule into battle, no longer can your stud or dude with all the useful attachments be Pistol Whipped out of the fight before they even get to do anything.
Pedro does come with a downside though. Your dude's value is reduced by three. I believe at the time this card was designed, Paralysis Mark could still target adjacent locations and was seeing a lot of play due to how powerful it was, so lowering your dude's value by three was a big deal. While it is definitely still a problem if your opponent has brought Shotguns, spells that target grit or value, or (hilariously) Forced Quarantine, in many matchups it simply won't be relevant. Even when it is, you can effectively negate the downside by attaching your Pedro to a low value dude who isn't going to care about the value hit anyway. As such, the downside isn't much to worry about as long as you're careful.
If I'm running eights I'll almost always bring at least two copies of Pedro in my deck. If it's taking advantage of the Horse or Sidekick keywords or wants to play and cycle cards quickly (e.g. Dead Man's Hand) then I'd increase that number.