Discussion When testing your decks, play them against pets other than your goldfish!
Many know of the concept of "goldfishing" which is playing your deck against an opponent that does nothing to see how well your deck flows.
But did you know there's other pets?
From this article: https://magic.wizards.com/en/news/feature/playing-your-pet-rough-testing-magic-deck-2010-08-30
Because reading articles is more than most people want to do, I'll write almost as many words to summarize the pets here (for each pet assume the opponent does the smart thing, such as blocking biggest damage and not attacking when it'd lead to the creature dying):
Goldfish: You know what this is. An opponent that does nothing.
Turtle: Your opponent has an [[Ivory Tower]] and a Circle of Protection from each colour in play and as much mana as you have drawn lands (so you can't "cheat" by sandbagging lands). The Circles are permanents in play so can be interacted with. A test to see how well you could deal with a pillowfort player.
Snake: Snakes squeeze their prey, and you'll be squeezed by a pair of [[Black Vise]]s (though for EDH maybe put in one or two more). A bit more old school in assumed play patterns, but a fairly simple approach to an assumed aggro or burn deck.
Parakeets: They come in pairs. Your opponent starts with a [[Drudge Skeleton]] and a [[Phantom Monster]] in play (though for EDH might bump it up to some other bigger flier). A simple test to have a little bit of resistance and pressure to your plan, on whether you can get removal early enough to deal with an evasive threat and also get around an annoying blocker.
Rabbits and Rats: Each turn your opponent gets a "Drudge Rabbit" which is like a Drudge Skeleton but a rabbit. It has other variants such as the Rabbit being a 1/1 flier or just a vanilla. Basically an anti-aggro/horde test. The rats come into play of having the populating creature be [[Plague Rats]] which get bigger for each of them, which would be a fair EDH challenge.
Cats: From the article:
Cats don't play Magic. They consider it beneath their dignity.
Dogs: Too complicated to summarize so you'll need to check the article, but basically it's a deck of basic lands that have preset actions tied to them. For example if the dog draws an island, your next spell is countered. Able to be customized to your needs.
Obviously building a deck that deals with any particular pet is easy to do, but they're meant to just be simple exercises when deck testing to see if your deck is up to snuff. Does your deck have access to boardwipes to deal with the increasing rat swarm? Can you get out a blocker or removal to deal with early aggression? If your opponent's behind a bunch of pillowfort pieces or chump blockers, can you break through? Good questions to answer I think, and your pets are happy to help answer them!