Let's talk about Hunts - a detailed rundown of their assumed mechanics

[Vandesaan]

Nuny's does makes some sense if you connect:
A. Sizable/Large Scale Battle = lots of enemies
B. FATEs scale based on how many participated and if it was completed last time = "Battle" FATEs have more enemies when completed by larger groups
A + B = Nuny may spawn if a ST FATE scales up high enough.

That logic though, is why we struggled for so long with Nunyunuwi. We let FATEs fail, because we were busy making them large-scale battles. Only once we stopped letting any of the FATEs fail did Nunyunuwi actually spawn. That's what I mean in that the hint was misleading and that it made no sense in terms of what the trigger actually is.

I still hold that Senmurv is going to be an either
a) really long time variation version of Nunyunuwi that requires an hour of no failed FATEs or
b) Nunyunuwi Extreme, meaning that there's going to be way more than an hour of no failed FATEs required.

If we group up the old world S ranks into categories based on their triggers we get:
Gathering: Croque-Mitaine & Bonnacon (triggers on successful gathering)
Item acquisition: Agrippa the Mighty & Minhocao (triggers when a crystal or shard hits someone's inventory)
Weather: The Garlok & Laideronnette (meet X weather conditions for Y period of time)
Moon phases: Mindflayer & Croakadile (meet X moon phase condition after Y period of time has passed)
Player buff: Nandi & Brontes (permanent buff from a summoned minion and apply a buff from eating a piece of food)
Player health: Mahisha & Safat (player dying and player almost dying)
Fishing: Thousand-cast Theda & Zona Seeker (acquire fish)
Leves: Wulgaru & Lampalagua (initiate leve)
FATEs: Nunyunuwi (do not let any FATEs fail or expire for one hour)

Now out of these categories the ones we know have been reused to far is Player buff (Kaiser Behemoth), Gathering (Gandarewa) and Item Acquisition (The Pale Rider). We've confirmed no global spawns happening like with Moon phases and with Weather. Assuming that there are no trigger pairs this time around, we're left with the following categories that are either reused or not used at all:

Player health
Fishing
Leves
FATEs

Since we've classified three out of six new S ranks, we can either assume these remaining four are being reused, or that there's been reinvention going on and new categories have been made. Data suggests that the implementation of the hunts this time is kinda lazy because of every single new A rank having the exact same respawn timer and not a fluctuating timer per mark like last time, so we can put in doubts about whether they did any kind reinvention. Let's go over the remaining three:

Leucrotta mentions smell of blood and the evolution it has undergone. This is kinda vague, but since we've got four categories left, let's start excluding some of them. There are no leves in Azys Lla, so that's out of the equation. I also highly doubt it is FATEs, mostly because it's the final zone you get to and it is a giant pain to get between all the FATEs. Player health is feasible, but let's look at Fishing for a second. The fish Lungfish has a wording similar to the evolution lines about Leucrotta, meaning that it may have been the preferred food of the cat throughout it's evolution? There might be something there, or we're looking at either a reinvention or Player health. Fish theory for Leucrotta has been debunked by spawns way outside of Clouds weather, keeping it here for historical purposes though.

Bird of Paradise is a tough one. The hint speaks of it loving the flesh and blood of beasts/animals, and would come swooping in if it heard them cry/scream. It might here well be they reinvented something or made a second S rank in the Item acquisition category, speaking of Dhalmels in particular. There's a particular leve in Sea of Clouds which is interesting, "Two Birds, One Culling", which has you bait out predatory birds using animal meat. Sounds familiar? This would put it in the Leves category. It might also be that players are beasts to the bird and that the sound of us screaming as we almost die like with Safat or dying like with Mahisha is what causes it to show up, putting it in the Player health category.

And then there's Senmurv. It hunts the hunters who are currently hunting. I've spent an unreasonable time looking at the available fish in The Dravanian Forelands, assuming that a fisher is a hunter, and I can't see any connection. Senmurv shows up at any hour of the Eorzean day, which could put it similar to Zona Seeker which just requires a Glimmerscale during the right weather. There are plenty of fish like this, but none of them specifically match all the recorded timeframes of Senmurv being killed. That leaves us with FATEs or leves. Senmurv continues to spawn on Excalibur without us knowing why, but it has only really spawned on weekends when there are way more people on, possibly doing FATEs. It was only after the 50% increase to FATE experience we started seeing Senmurv show up on weekdays. It's a flimsy connection, but this was also the pattern of Nunyunuwi way back in 2.x for Excalibur. Leves is also a possibility, but there are no leves that particularly caught my attention, so it may be any leve like with Wulgaru and Lampalagua.