Look like Carnarvonia-araliifolia commonly known as the red oak, red silky oak, Caledonian oak or elephant's foot

Not at all likely. Unless OP is in the rain forests of Queensland, Australia. Did you use one of those id apps?

Could be an apple.

Can you describe the plant you took the cutting from? Was it a vine something like Celastrus scandens? Apple was my 1st impression also.

Carnarvonia araliifolia has compound leaves with different sets of leaflets having different petiolule lengths. It's petiolules are thin. The OP's plant has simple leaves in an opposite arrangement at nodes along the stem. The petioles are thicker, almost winged, and the stem is conspicuously thicker. I use google lens for leads and if a result looks right I still have to scrutinize all the details to verify it. Google lens gave me Liatris spicata for the other post also, but the red stem bases seemed not typical, so I had reservstions, because many plant species have a similar look at that stage, though I don't see anything that would rule out Liatris spicata.

Carnarvonia araliifolia

Jay, the cutting I was rooting was cannabis and this other thing just sprouted from the Pro-mix in the pot. This is definitely not that. :)

Here is the seed that detached from the base of the seedling. It split in half and I’m showing both the inside and outer shell. Do you think it was an apple seed?

Sorry for the late reply. I don't think your plant is any fruit bearing tree in Rosaceae, floral? The petioles don't look right for those. It could help if you take a photo of the side of the plant that shows the leaf arrangement and other details. Were you taking cuttings from a mother plant? I had fun growing Cannabis, and I really got into it. I made a supercropping net and was getting into the mathmatics of pruning for higher yield. If I had more area to garden with, I would definately have a few Cannabis trees. I'd get in trouble growing it in my front lawn, but I have an Illinois Bundleflower next to the street that produces DMT. I haven't used it for that purpose. I grow it for it's ferny foliage and powderpuff flowers. Illinois has some of the best mmj, but when I grew a hybrid of Malawi Gold and Highland Thai outdoors it rivaled the stuff from the dispensary. A true shame that the powers that be still haven't reclassified Cannabis and still outlaw Psilocybe and Salvia divinorum. Ethnobotany is so interesting! Fascinating that the human brain has chemical recepters that exist to bind with THC.

I appreciate all of the input here, and everyone loves a good mystery. :)

To be very clear, I took a cannabis cutting (Blue Northern) and had it in a pot of Pro-mix to root. It’s been legal here for years but NY has screwed up the dispensary licensing requirements so badly that we still don’t have dispensaries. So I checked the cutting for roots the other day and found this seedling growing along side it in the pot. I’ve used Pro-mix for decades and never had anything sprout in it before, so I was curious about what it is.

I tried to get more photos — I’ve kept the seedling in water, but the seed is drying naturally.

Google lens says Dog's Mercury, specifically Mercurialis perennis. AFAIK that species only grows in a small section of Long Island. It's weird because I've only known about the existence of Dog's Mercury for a couple weeks. It sounds like a proper name for a ubiquitous weed? A cousin to 3 Eyed Mercury in the same family Euphorbiaceae.

I'm very familiar with Dogs Mercury and Annual Mercury. They're both very common all over the UK. While the leaves are similar I'm not convinced by the seeds.

I love those common names. I just learned that Stachys species are woundworts. But you probably call them hedgenettles too. Isn't Garlic Mustard also Jack By The Hedge?

I’ve never heard of any plant called Mercury, so had to research. Dog’s Mercury then. Woundwort doesn’t sound as bad to me as lungwort. Jack By The Hedge sounds too good for Garlic Mustard. :)

Thanks everyone!