If you put a Trickster's Elk on an opposing creature to shrink it down to a 3/3, and then the creature dies later on, you are the one left with the original 3/3 Elk on the battlefield, not your opponent
The original iteration of companions came with Ikoria: Lair of Behemoths, and on release they were placed in the sideboard and could be immediately put into play from there by paying the companion's mana cost
You'll often cast Hospital Room first since it doesn't need as much set up, but that makes this an Enchantment that gives a decent buff and then reanimates something small late
Rather than giving up on making aggressive decks work, design pivoted to ideas like extra combats to improve these strategies
With Pongo, 'Old Rascal and Peter Pan, Never Landing, you can stick cards on your board early and generate lore unchallenged, every single turn
Stat-wise, its decent: a 4 mana casting cost and another two mana to put it on