Any idea what this is?

[Benjishore]

It was tiny and chunky, then it got all tentacle-ey.

[stefan m.]

It appears to be some kind of hideously etiolated mammilaria, likely grahamii.

[DaveW]

I am afraid cacti are not plants for the interior of rooms. They are high light intensity plants and need to be on a windowsill or somewhere similar.

You say "it was chunky" and that's how it should be. The "tentacles" are etiolated growth due to lack of light, but even the bottom growth is a little "open" and not as densely spined as it should be. In the growing season you could propagate from it by cutting the "tentacle" off one head and that will encourage the plant to sprout new growths with the plant in better light, then when the sprouts (known as offsets or pups) are large enough cut them off and re-root them to get a normal plant again.

Not sure if it is M. grahamii as it is hard to tell both from the photo and the plant now, but your plant should look more like this when grown properly in full light:-

http://tucsoncactus.org/cgi-bin/db/dbma ... ew+Records

[Benjishore]

Ah geez,

so it seems that the sunlight was not enough, guess I'll place it in shared common patio area now.

Thank you for the info and the help.

[stefan m.]

Wherever you place, feasible growing side needs be east/southeast or west/southwest for younger and smaller plants like mammilaria.
South is preferable when they get older and bigger.(unless you live in the south hemisphere, where it would be reverse)