Echinocereus pentalophus also known as ladyfinger cactus is the cactus that is drooping and rectangular . Your form is thin, maybe etiolated, but I hear that there are different forms of it. It is native to the Rio Grande Valley.

Thanks Mara. I think the thin one is Echinocereus poselgeri (?) Sometimes called Pencil cactus or dahlia cactus. It's only bloomed once for me.

there are a bunch of these, some are thick and fat, some are thin.