While the card-drawing mana rock has seen some fringe Standard play, it has been immediately adopted into a lot of Commander decks built around big creatures, which is driving its price
it even rebuilds well after a board wipe since you just need to play this and another cheap creature to get four or five counters at once
A 'power 5' themed deck could take a twist of just running creatures that meet that criteria, with cards like: Terastodon Archetype of Endurance Ruric Thar, the Unbowed Spearbreaker Behemoth 8 Arna Kennerud, Skycaptain Master At Arms The Human Knight Esper Arna Kennerud, Skycaptain is an absolute powerhouse that doesnt require any fancy combos or game changer cards, instead, you can just beat face with a very strong creature and a ton of auras and equipment
The old black sword (called Sword of Yawgmoth) gave +2/+1 for 2 (this is back when artifact equipment worked just like creature enchantments)
Rin and Seri, Inseparable can make especially good use of this effect, given that Mirror Entity counts as both a Dog and a Cat
Most players cottoned on to the synergy Cauldron Familiar has with Witchs Oven early on, and the two went on to define their very own deck type, the notorious, and aptly-named, Cat-Oven