I just discovered my cats’ new favorite toy: I was eating a bunch of grapes while watching TV. When I finished the grapes I was left with a little tree-like bunch of stems. For some reason I threw it at one of the cats, who immediately started throwing it up in the air, chasing it, and batting it across the room toward the other cat, who joined him in his little game.
Got any other impromptu pet toys?
I owned Puppy-Cat, a lilac Siamese rejected by his mother and raised by a mother dog. He loved to retrieve balls. One day I threw a wadded up cigarette pack into the waste basket and missed. Puppy-Cat was right there, retrieve the pack and brought it back to me.
Thus begat Puppy-Cat’s favorite game: Retrieve the wadded up cigarette pack.
One of my golden retriever’s favorite toys is a kevlar work glove. I put it on my hand and we wrestle around and she’s allowed to bite me.
She also loves gloves in general. When we walk in the winter she won’t go inside unless I let her take my glove off my hand, then she darts in and waits for me to come take it from her and yell at her. And rub her belly.
Is my dog in to s/D? I guess that’s a dog’s whole schtick, really…
My cats’ favorite toy is this long wide gold mesh ribbon I had saved from a gift wrapper. Just drag it across the floor in front of them and they lose their minds.
Toy that’s not supposed to be a pet toy: The only thing my rabbits will play with (with “play” meaning picking it up in their mouth and flinging it, versus just chewing it up or ignoring it) are those plastic keys intended for babies.
I have one cat who will bat around anything small sized. She once fell in love with a latex ear plug and hid it away, only to take it out when all the other cats were sleeping. But the most long-lasting is the red plastic ball Christmas ornament tied with a bit of ribbon. I think that ball has been around for at least two years of catball action.
Also there’s a huge love of toilet paper rolls… either with or without the toilet paper attached.
Is a laser pointer a “toy” or an “enemy/threat” to a cat? I’ve never seen a cat not driven crazy by one.
Also, wadded socks or gloves, and my cats loved small pill bottles, especially ones with pills still in it so it rattled.
With Demon, a string with a knot in the end to fight me for total control. ( I don’t bleed too much cause I am quick )
Pet toys are a billion dollar industry, but my guys’ favorite thing ever?
Ponytail elastics & crumpled paper balls for the cats
Empty plastic jugs for the dog
Silly critters 
Laser pointers for just a few minutes, then they wise up. yeah, all seven of them are smart. Well, a couple won’t even play. that is smart… right?
Oh. Oh, & moles, mice, undead birds, they have a lot of favorites…
My dogs enjoy playing with empty milk cartons and 2 liter soda bottles. I kinda half crunch them down, maybe put a few pebbles in them so they rattle and toss them in the yard and watch the fun!
My horse used to like batting one around when hung in his stall, too. He also liked kicking a soccer ball around his stall and LOVED to kick it over the top of the stall wall at people.
During the two or three years in which we had exactly three cats (before my sister, rather insanely, added yet two more), each cat had their own Signature Toy.
Eldest cat Hailey (aka “Nurse Kitty”, because she insistently reminded Mom to take her medicine, and comforted each of us when we were sick) loved toy mice. When she couldn’t find it, she yelled for it. “My MOUSE!!” her meowing seemed to say.
Youngest cat Toby (aka “Target Cat”, because of the large target mark on his sides, which was a perfect petting spot) liked cat-sized soccer balls.
Neither fits the topic.
But middle cat Gizmo was into milk carton rings! He’d slide that darn thing across the room! Very fun to watch. You had to get one of those rings that would come off and stay attached, though-- he was utterly uninterested in the “unwrap” kind. (This would, of course, directly influence our milk-buying habits if we needed a new milk ring for him).
Doyle (the cat) is kinda odd. For one thing, he loves to eat chips – potato chips, salsa chips, Doritos, you name it. I figured he just sees them as another kind of dry cat food, so to test this theory I dropped some Cheerios in front of him one day while he was lying on the table. He sniffed them, pawed at them, looked at them quizzically for a few seconds … and then perked up. “Oh, I get it!” He swatted them off the table and ran around like a madman chasing them.
Weirdo.
The other cat likes to play fetch with balled up little paper/foil wrappers.
My cat loves the little yellow foam earplugs - she treats it like it was prey. She is adorable trotting across the room with her fat tummy wobbling holding a little yellow earplug in her mouth. 
Butcher’s paper. One of the companies I order stuff from online packs their boxes with long & wide sheets of paper. I left it out the first time and discovered that my cats love it. They jump on it, run through it, whatever makes it make noise. I throw it out when it gets too shredded.
My cats knock the leaves off my plants and chase them around the room and throw them up in the air and so forth. Then they abandon them. So, no, they don’t eat my plants but they do destroy them.
Back in the day I had a cat that loved empty cigarette packs, like Annie-Xmas’s cat. I wasn’t the world’s champion housekeeper in those days and when we moved, under the sofa we found a lifetime supply of wadded-up cigarette packs. Apparently Buffy was saving them in case of a shortage.
12-pack cardboard boxes for soda. Caelan will play with a Coke Zero box for days until it is smashed flat.
Lucy: Ponytail elastics and string. Heaven forbid you wear pants with a drawstring waist - more than once I’ve been woken by a furball cat attacking my pants.
Mayme: Loves her babies. She has a mouse toy that she carries around, but lately her favorite is a small McDonald’s pink teddy bear toy. She carries it in her mouth, meowing so proudly, showing it off. She’s also a sucker for washcloths - literally. She will bring me a washcloth for me to dampen, then she’ll curl up chewing and sucking on it.
The there’s Dot. Dot loves twist ties, ponytail elastics and milk bottle rings. Her favorite non-toy toy, though, are Werther’s candies. She will get into the candy dish, knock one out, and chase it all around the house. When I put peppermint discs in the candy dish - completely ignored. Halls cough drops she’ll bat under the stove. But Werther’s with their gold wrappers? Dot heaven.
Brown paper bags were a Disneyland ride for several of my cats, especially my very mellow and long-elderly shorthair. I’d drop one SO shorty on the floor when unpacking groceries and pick up the battered remains a few hours later. She loved to play tiger in there - tap a finger on the bottom or side and she’d spend fifteen minutes furiously attacking.
Not really a toy, but my second Golden was EXTREMELY mouthy, as retriever breeds often are. I can’t remember how it started, but every night I would give her the shirt I had been wearing and she would roll it up into a ball and happily spend the night doing that sort of gentle, vibrating mouth-cuddle on it. Never so much as snagged a thread, nor did she slobber excessively on them - just loved them, for the mouth-cuddle and because they tasted like her Earthly God. I had to stop when I got my boy Dane, because he also liked them… for the ten minutes they lasted. I had to make a point of giving her shirts when she had the room to herself instead of overnight.