Elves
Prominent elf historian J.R.R. Tolkien had a lot to say about the nature of their reproduction and sexuality — for example, elves in Middle Earth don’t celebrate their birthdays but the days on which they were conceived, and “the union of love is indeed to them great delight and joy.” However, he was pretty tight-lipped about menstrual cycles. Shock.
Still, on the subject of elf biology, he did stress that men and elves were not so different from one another that they could not interbreed and make gorgeous half-Númenórean king babies. Humans get periods, so if elven reproductive organs are compatible, then they will proably also get periods. This also goes for any franchise that features cross-bred elves as well—Dragon Age, Dungeons & Dragons (which also has half-dwarves, so they’ve probably got periods, too), and so on.
Of course, most iterations of elves are immortal beings who do not track the passage of time in years, so it stands to reason that their cycles might not be monthly in the same way that human cycles are. But how about the marginalized inner-city Dragon Age elves, who’ve all but lost the ancient magic which guided their ancestors? They’re probably stuck on month-to-month menstruation like the majority of us. Bit of a nasty shock for Merrill when she moves to Kirkwall and ends up on the alienage’s collective cycle, I bet. Though, hey, you never know with blood mages.
Werewolves
Really, isn’t lycanthropy already a very thinly veiled metaphor for menstruation anyway? (Except those times when it’s an AIDs metaphor, of course). Think about it, though: once a month your body undergoes a dramatic change, and even when you’re fully aware and able to prepare yourself for the negative side-effects, everybody in your life gets weird and makes your pain all about how it’s going to affect them. They’ll sit around all judgmentally and say stuff like, “You’re in a bad mood, is it that time of month?” or sometimes, “OH GOD STOP EATING ME I’M YOUR FRIEND SOMEONE HELP.” Ugh. People are so insensitive, am I right?
Ahem, anyway.  Most non-human mammals have what are known as estrous (or oesterus) cycles from the time they are sexually mature until their death—we know it colloquially as “being in heat.” As monoestrous creatures, wolves only get one cycle per year in order to give their offspring time to survive the winter. During proestrous, or the time before oestrous begins, female wolves do occasionally experience bloody vaginal discharge. However, this is not menstruation—wolves are actually able to reabsorb their uterine lining back into their bodies if they are not pregnant by the end of their breeding cycle. Humans, in contrast, shed that lining, which is what causes periods in the first place.
So we can imagine that determining whether or not werewolves get “periods” depends entirely upon how much their wolfiness actually intercedes upon their humanity. If they are fundamentally homo sapiens who just happen to magically become wolf monsters sometimes, they probably still have to carry spare tampons around like everyone else. But if being bitten affects a werewolf on a deeper biological level, then it’s possible that it could alter them enough that they’d only experience a brief period of ovulational bleeding, or spotting, every year.
Of course, having a single estrous cycle would also make you completely uninterested in sex, except for those two weeks a year when you’re ovulating. So… yeah, tampons it is.
Mermaids
At the heart of this question is a bigger discussion about the nature of merfolk: are they fish, or are they mammals? If we imagine that the human-like breasts on most mermaids suggest a mammalian origin (and not just that a whole bunch of sailor dudes wanted to draw sexy fish ladies with sexy fish boobies), then we might look to dolphins as their nearest real world equivalent.
Dolphins, like wolves, do not have a menstrual cycle but an estrous one. Unlike wolves, however, dolphins have polyestrous cycles, meaning that they are fertile multiple times throughout the year. They’re also totally down to have sex when not in heat, too (and other than primates, they’re the only non-human species to do so), and often engage in long foreplay and masturbation games. But no, they do not shed their uterine lining, so they do not have periods. Which is for the best, really. There’s a reason most women don’t swim when they’re menstruating.
If we’re working with the assumption that mermaids are actually evolved from fish, though, then their reproductive systems and breeding habits are going to be completely different. They still won’t have periods, of course, but they also won’t be engaging in penetrative intercourse. Instead, the female will release her eggs in a spawning ground, and the male will fertilize them at a later date. Aw, man. Is it getting hot in here?
(Vampire teabag cozy via dreadfulgirl on Etsy)
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