male human to gryphon TFed by a alien symbiote (like venom from spiderman)

Ooh, I second dragonmanmike's idea!

age regression, pangolin.

Women to black widow due to misterious blow dart.

Awesome! Got a good sketch out of this one. Thanks :3

Welcome. Just glad my over active imagination could be of asistance.

male human to chimera anthro female due to exposure to multiple stands of agressively mutagenic animal DNA (lab-accident type scenario)

woman to jackal due to egyptian curse

or

woman to dog due to bite from lab test subject

Ooh... I like that first one. Thanks :3

male to bird through ingestion of some sort of bioengineered egg

woman to raccoon from (go with a classic) lab experiment gone wrong

Climbing Tree, Pangolin, Female

Woman to meerkat through children's medicine(?)

Nagas and humans working together in a zombie apocalypse.

Peryton?

Cursed water, female, otter

As the tainted fertilizer transformed Jack Green into a giant earthworm, he wondered bitterly if he should have chosen manufacturers by good reputation rather than low price.

Ooo! Ooo! Ooo! Greg, while attending a strange and unfamiliar carnival, wins a stuffed red panda at one of the carnie games. He gives it to his love interest, Linda, who thinks it's absolutely adorable. The little doll is magical, however, and as she hugs it close to her chest she begins to change ...

A Dhole

Not sure if this qualifies as horse, but how about some kind of Greek artifact makes a guy transform into a centaur?

I still say there should be more skunks around. Perhaps even male human to female skunk. Perhaps from a spray by a cute male skunk...

...Bias? What bias? I'm not biased at all! ^_^;;

Could always do a mix of anthro and nonanthro forms, someone about halfway between... They'd look good in a collar, I'd bet. ^_^

Guy into a pokemon by a damaged pokeball

A witch turns a man into a peacock for being excessively flirty.

That's a good one.

Hmm...
Girl to tiger from Tan In A Bottle (since it tends to make you orange...).

Girl to rabbit from sexy bunny outfit.

Guy into platypus by a hit from the Random TF Gun (tm, patent pending).

Male to female guard dog for trying to mug a mad scientist.

An astute English gentleman turns into a mongoose due to cursed tea leaves. Top hat and monocle are a must.

Uh, science teleporter mistake, female, and kangaroo? Why? Because 'The Fly' proves that teleporter problems are awesome for messing folks up.

Or cursed spikey collar, male, and cerberus, because cerberii are cool.

Um, woman, owl, some sort of mystical jiggery-pokery.

Man to lizard through regenerative healing experiments (only transformed in places that had to heal/grow back) (Ala The Lizard from Spiderman)

Young girl on a farm gets into her father's experimental bull growth hormones.

girl to beast via mask triggered by anger (ala billy and mandy: mask of the beast)

Woman visits stepford-like town inhabited by badgers, the air is tainted with a bioreactive agent, she transforms/is brainwashed the longer she stays

A woman takes her own super serum which is intended to allow her to consciously alter her own anatomy, however, as she makes changes, the changes start to alter her mind, leading to a regressive resulting form (start out with some fairly normal sparkle-dog additions that just about anyone would want, such as wings if you'd like, but as the changes go on, she takes on more predatory features if you need a base species to jump from, I'd suggest T-rex.)

I may return with more...

male to scarab after stealing a golden scarab from a acient tomb

male to anthro herm cow from drinking from the carton when it said not to.

Lets see, Paliontologist into Dinosaur from a cut of a fossil,

Male or Female into Buffalo from cursed Indian Arrow Head

Sprinters into Tortoise and Hare from tainted running water

Scuba Diver into dolphin, orca, or whale while testing a new sonar equipment

Female to sphinx after reading a cursed book of riddles.

Person to ferret by collar. Mabey a witch's familiar. :)

I dunno if this goes into the cat category, but perhaps a displacer beast m->f with a cursed relic as the trigger. Could be interesting.

Edit: Failing that, perhaps a halfway transition to a Phoenix

Daaaaaaaang. Next time I get writers block, remind me to just put up a journal. O.O So many ideas!!!

poison dart trap TF a pair of explorers into M&F quetsuqaltle or just naga

Group of beachgoers, otters, nanotech collar/choker; maybe a subtle nod to Ollie Canal?

rabbit, girl, self-administered injection.

"Headhuggers", small cute possums and raccoons that latch on to people's heads and cause them to change into anthropossums/coons. Said victims then toss the huggers at someone else :)

Armadillos/Pangolins are the source for leprosy in real life. Contact could cause people to change into them.

Leopardosy: a condition spread by touch that causes people to change into leopards.

Leopardsy would be more like parts of their body fall off and turn into leopards.

Perhaps some kind of cursed/magic fruit to turn 'em into an otter or raccoon with a bit of bellypudge on 'em...
and maybe m to f, for a bit of growth to more than just her belly. X3; I dunno.

Bad milk, Male, Cow

Whinie the Pooh(book), Male, Rabbit(dont see too many male rabbit tfs)

Anger Issues(Stress ball?), Female, Eastern Quoll(cousin for the Tasmanian Devil but smaller with more anger issues) .

Ill come up with more later.

Woman into Chicken caused by breaking an egg?

Or

Woman and Man couple body swap by dressing as each other for a costume party.

Or

Woman or man into Draenei/Night Elf/Troll/Some WoW character by playing too long

Or

Woman into mouse by eating cheese

Or

Woman into pig by making fun of a fat person

Or

So many possibilities!

Oooh! To clarify my otter idea: someone dumps a packet of powdered nanobots into a hot tub or something, the water turns faintly pink, and people get in after making excited comments. Then, they sit around in the hot tub, enjoying the change, making wisecracks, and suchlike.

Young apprentice sneaks in after dark to use his/her master's taiko drums, tf to anthro monkey oni.

Inflation into mould, male, elephant.

Ancient Greek hero Jason into anthro ram after obtaining the Golden Fleece?

girl to naga by an ancient statue

How about you do someone turning into my Character Dudeguy15, or someone using Dudeguy15's TF ray.