Agree, Notocactus (Parodia) buiningii

according to Britton & Rose (First published in Cact. 2: 125 (1920) Acanthocereus subinermis is a synonym of Acanthocereus tetragonus

Mammillaria bocasana would be my guess

Another one to consider, Thank you 7george!

Thank you for your suggestion, Tom! it was indeed bought from a retail nursery, no tag. Alensis is a possibility, I am considering gaumeri as well. Guess I need to wait some more and see how it grows and possibly blooms..

Thank you Anttisepp! it does remind of pachycladus, but at that age it should be already blue/bluish. Mine is as green as a cucumber.. Will keep growing it and observing if anything changes. I like the plant, name or no name

Hi, I have a columnar cactus that I cannot identify for some time. Guessing it might be one of the Pilosocereus species. Any help will be appreciated.

Looks like Mammillaria nivosa to me
https://llifle.com/Encyclopedia/CACTI/F ... ria_nivosa

Agree with anttisepp about M.haageana. Both perbella and formosa at that age are globose with flattened apex. Your plant is elongated, which is more often with young haageana (yours could be M.haageana ssp.elegans). Flowers could help to narrow it down.

You are right, it is difficult to say at that age. P. pachycladus is possible, but I would also consider P. languinosus or P. glaucochrous.
you can check photos on https://succulentalley.com/pilosocereus/

Could be Mammillaria pringlei

First one looks like Stenocereus pruinosus

@nachtkrabb I am curious about Matucana fruits you mentioned. Have you seen seeds inside them? Did you have a chance to sow those seeds? From my experience, sometimes cacti produce fruits after being pollinated with other unrelated random cacti species that bloom nearby at the same time. The process...

You could not find it under Opuntia species because even being an Opuntioid, it is not Opuntia per se. the correct name as MrXeric mentioned is Miqueliopuntia miquelii. "Miqueliopuntia is a monotypic, genus of cactus in the Cactaceae family, containing a single species, Miqueliopuntia miquelii ...

possibly Mammillaria backebergiana ssp.albispina