Who do you think was the mistress of Aegon IV that was caught in bed with Terrence Toyne and afterwards executed? (We know who the mistresses of Aegon IV were, well, at least, the most important ones, but there is never any hint of which one could have been Toyne's lover.) I know there's not much information to reach a conclusion here, but still, do you have any theories?
Ah, the nine mistresses of Aegon IV (divorced, beheaded, died… wait, sorry, wrong lustful king who went from handsome and charming to bloated and unpleasant)… let’s list them and figure things out, shall we?
Bellegere Otherys, the Black Pearl of Braavos
Melissa (Missy) Blackwood
We know Terrence Toyne’s lover couldn’t have been Serenei of Lys, since she died giving birth to Shiera Seastar. Cassella Vaith is almost certainly the old Lady Vaith who Dunk managed to offend the year he was in Dorne, so not her either. Bellegere Otherys was not a Westerosi, and the mother of the next Black Pearl of Braavos (her great-great-grandaughter is the current one), so we can rule her out too. Barba Bracken was put aside for Melissa Blackwood, and if either of them had been executed Hoster Blackwood surely would have mentioned it when he told the story of the renaming of the Teats hills, so they’re probably not the ones either. Ditto Bethany Bracken, since the Toyne scandal has never been mentioned in the Blackwood-Bracken feud, nor in relation to Aegor Rivers (son of Barba), who is noted to only have a grudge against Brynden Rivers (son of Melissa), and not against his father (for killing his aunt/cousin/whomever).
So, that leaves Falena Stokeworth, Merry Meg, and Jeyne Lothston. Now, with those three, we have to work with the timing. We know that after Terrence Toyne was executed (by dismemberment), his two brothers did their best to kill Aegon. They were stopped by Aemon the Dragonknight, though he died in the process.
But we know Aemon did many things during his brother Aegon’s rule, including defending his sister Naerys, Aegon’s queen, from the slanders of Ser Morgil (btw this is definitely a Lancelot/Guinevere reference). He also entered a tourney as a mystery knight in order to name Naerys Queen of Love of Beauty, and so that Aegon couldn’t give one of his mistresses that title. And per the wiki article on Naerys, the AWIOAF app says she did not long outlive Aemon… but we also know that her children Daeron and Daenerys were born many years apart. So, from all this evidence, I would think the Toyne scandal (and eventually Aemon’s death) took place fairly late in Aegon’s rule and procession of mistresses.
Therefore, it seems most likely to me that the mistress in question was Jeyne Lothston. The Lothstons have a bad reputation, too, so that scandal would seem to suit their dark history.
Though it is possible Toyne’s lover could have been one of the early mistresses, or even Bethany (since I don’t have that much evidence to exclude her); nevertheless, Jeyne is my bet. Maybe if we’re lucky, the World of Ice and Fire will help confirm the details of this story.