When I first started playing D&D, familiars used to give their Wizard bonus hit points equal to the familiar’s own hp.
For instance, if your familiar had 2 hp, you got a bonus 2 hp. However, if your familiar died or was killed, you lost double its hit points. At the time, Wizards had a d4, so losing your familiar could literally kill you at low levels.
When did familiars stop giving bonus hit points?