Spider-Man has been represented as multiple non-human creatures:
Has any other Marvel character in any continuity been represented as a non-human character/creature that is not the same as their first representation?
Deadpool has been portrayed as several non-human characters.
Among them is a dog — Dogpool from Prelude to Deadpool Corps (2010).
In Deadpool the Duck (2017), Deadpool fused with Howard the Duck and became Deadpool the Duck.
Deadpool was also portrayed as a dinosaur — Deadpool Dinosaur (amalgam with Devil Dinosaur) in Deadpool Kills Deadpool Vol. 1 #3.
Some of the other non-human Deadpool variants (full list here):
Kang the Conqueror has been represented as multiple non-human creatures (members of the "Council of Kangs") in Ant-Man and the Wasp: Quantumania (2023 MCU film).
The above scene took inspiration from the "Council of Cross-Time Kangs" in Avengers #292 (1988).
Doctor Strange met a variant of himself, Doctor Stranger Yet, who was a warthog, in Doctor Strange Vol. 2 #25 (1977).
(Thanks to Bjorn Eriksson, for mentioning this in a comment. Image is from: https://www.supermegamonkey.net/chronocomic/entries/doctor_strange_2428.shtml)
Captain America (Steve Rogers) has been portrayed as a werewolf — CapWolf in Captain America Vol. 1 #405 (1992).
Sam Wilson as Captain America has also been portrayed as CapWolf in Captain America: Sam Wilson Vol. 1 #3 (2015).
A variant of Loki, “Alligator Loki”, made its appearance in Season 1 of Loki (2021 MCU TV show).
Loki has been depicted as a cat in an alternate cover by Skottie Young for Thor: God of Thunder Vol. 1 #1 (2012).
(Thanks to Trish for mentioning this in a comment.)
Dino-Thor, a variant of Thor who is a humanoid dinosaur and a member of the Thor Corps, appeared in Spider-Island #5 (2015) and Secret Wars #7 (2015).
A variant of Thor who is a humanoid boar and member of the Thor Corps appeared in Secret Wars #4 (2015).
Thor appeared as a frog in Ultimate Spider-Man (2012 animated TV series) S01E09 "Field Trip".
(Thanks to Clockwork for mentioning this in a comment.)
Thor was portrayed as a cat: Cat Thor (Earth-53147) in Unbeatable Squirrel Girl (2015).
Bruce Banner / Hulk was portrayed as a dog: Dog Bruce Banner / Dog Hulk (Earth-53147) in Unbeatable Squirrel Girl Vol. 2 #17 (2017).
A variant of Rhino that is a literal rhinoceros appeared in Spider-Man: Across the Spider-Verse (2023).
Norman Osborn / Green Goblin has been portrayed as Norrannosaurman, a pteranodon and nemesis of Pter Ptarker (Spider-Rex).
Mysterio has been portrayed as an anthropomorphized car: Mr. Reo Speedbuggy (enemy of Peter Parkedcar / Spectacular Spider-Mobile).
Among many other Marvel characters (full list here), Doctor Strange has been portrayed as Croctor Strange / Steamin Strange, Mister Fantastic as Mooster Fantastic, Namor / Sub-Mariner as Sub-Marsupial, Iron Man as Iron Mouse, Steve Rogers / Captain America as Steven Mouser / Captain Americat, Black Bolt as Black Colt, Frank Castle / Punisher as Frank Carple / Punfisher, and Doctor Doom as Ducktor Doom in Larval Earth (Earth-8311, the home universe of Peter Porker / Spider-Ham).
Mysterio, as Mooseterio, also from Larval Earth / Earth-8311, appeared in Spider-Man: Across the Spider-Verse (2023).
Multiple Marvel characters (full list here) have been portrayed as apes and monkeys in Marvel Apes (2008, Earth-8101).
Marvel Apes Fantastic Four 2099: Doctor Strange, Jean Grey, Ghost Rider, and Nick Fury (as Nick Furry).
The Ape-vengers
Luke Cage, Giant-Man / Hank Pym, Wolverine, Spider-Man, Hulk, Captain America, and Black Widow have been portrayed as werewolves in Marvel Zombies/Army of Darkness Vol. 1 #5 (2007).
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Multiple Marvel characters have been portrayed as cyclopes in Ultimate FF #6 (2014, Earth-61422).
The Avengers (Iron Man, Black Widow, Thor, Doctor Strange, Captain Marvel, and Captain America), the Fantastic Four, and Doctor Doom have been portrayed as dinosaurs in Fantastic Four Vol. 7 #12-13 (2023).
Throg - Though he is a different character from Thor, he is considered a Thor analogue in frog form. Throg's powers are like Thor's. Simon Walterson, who got turned into a frog (Puddlegulp), finds a sliver of Thor's hammer Mjolnir (Frogjolnir) and uses it to become Throg, the Frog of Thunder.
Throg made a cameo appearance in Season 1 of Loki (2021 MCU TV show).
Definitely not the only one. Within the Spider-Ham universe alone, you have characters with different animal mutate versions:
Mary Jane Watson
Betty Brant
Namor the Submariner
Kraven the Hunter
The Watcher
Ant-Man
Captain America
Daredevil
The Hulk
Iron Man
Kingpin
Magneto
The Fantastic Four
