Can any of these fill your criterpedia?
you have to catch the critters legitimately for critterpedia to populate or use the NHSE save editor to change the flags to all caught, there must be a separate flag that indicates whether a critter has been caught, given to Mathers for the museum etc.
Poker can poke critters into pockets/slots and you can give them to visitors using wrapping but they do not flag as caught. You can donate to museum and they will display and set the flag to say Mathers already has the critter in the museum.
The action of catching something is linked to a trigger/event that must populate the Critterpedia, The flag must be detectable using a mem editor/cheat finder. I'm guessing each Critter's data is stored in nearby memory or table? We just need to discover where that is. There may be clues in the save file if someone can reverse engineer this?
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do you know if there's a way to do it via local play so i can have another switch over?
It already works having a visitor over so long as you poke prior to visitation and you don't close the session.
Make sure the chosen Critter(s) are spawning before your friend comes over.
Check in the Critterpedia whether selected/unselected are enabled by viewing the stats for the ones you have already caught. You will see disabled as no availability on the calendar, enabled as restricted availability say March to August (normal spawn) and Fully available (24/7 & 10x frequency enabled) Jan to Dec midnight to midnight.
They may stop spawning after depletion so I would choose 1 critter per spawn location, i.e. river, pond, sea, pier, tree stump, tree, flying around etc. If this happens, send you friend home and re-poke following same procedure.
Certain critters do need other factors Coelacanth (raining & 100plus critters caught), Snail (raining) Jewel & Citrus Bugs (tree stumps), Atlas moth (tree) , Sturgeon (river mouth) etc etc . If you aren't getting the desired spawns then there may be other factors that affect the spawn, i.e. location / environment / weather. The cheat will only work if these are in play.