The closest I came to having a 'pet' was the human-sized, sentient steamjack (using warforged stats) I had as a cohort in an IK campaign. He (it?) was a trusted ally through several tough battles, but got destroyed in the same session my PC was killed (and lost a level due to being
raised). Despite the obvious 'we can rebuild him - stronger... faster... more powerful' joke, the reality was, the PCs
couldn't actually do that and the mechkanic who could was many weeks journey away in a direction we weren't going. Since my character had lost his Leadership feat when he lost his level, I went ahead and took a different feat going forward.
However, the MOST memorable pet I ever saw was the Axewing (a somewhat broken, possibly erroneously typed creature from the Monster Encyclopaedia II) animal companion from my last Spelljammer campaign. She was quickly dubbed Empyria (after the Godbird in Dragon Quest 8) by the players because, well, she was a bird... and seemingly a god. Already a powerful, possibly under-CR'd critter in a normal campaign, she was death on wings in a game where aerial mobility was key. And lucky? That bird critted more often than not. Her master, a raptorean Cleric/Druid, rarely involved himself in fights, but why, when his animal companion was the party's best damage dealer?