Why do we call it masechet Beitza now, and not masechet Yom Tov, as was apparently once common? Unlike the parshiot, which are named after their first words, masechtot are all named after topics, except Beitza. This is particularly odd considering that today, many people actually call it "Bei'a" to avoid a word not appropriate for mixed company. So why not just call it "Yom Tov"?
See the beginning of Rabbenu Chananel on Masseches Beitzah, where he - or whomever copied the manuscript the printed text is based on - began it with the following rhyming ditty:
אתחיל מסכת ביצה בעזרת גדול העיצה
Maybe because "Yom Tov" just sounds like too generic a name? There are, after all, several other masechtos that discuss, and/or are named for, specific Yamim Tovim.