Appears to be a ciliate protist called Vorticella.
Image source: Wikimedia (Author: Frank fox, 2010)
Vorticella always stood out to me because of their long, narrow stalk they use for anchoring with ciliate-lined bell-shaped oral end.
Image source: MicrobeWiki (originally from Haw River Program)
Fun fact from Wikipedia:
The stalk is made up of the spasmoneme [also called "myoneme" above], a contractile organelle, with rigid rod filaments, batonnets, surrounding it. The coiled spasmoneme and batonnets serve as a molecular spring, so that Vorticella can contract. The cell body can move hundreds of micrometers in milliseconds. The spasmoneme is said to have higher specific power than the engine of the average car.
You likely mistook this for the more-commonly known rotifer due to the beating cilia at the oral end and tendency to anchor down, which are also both characteristics seen in rotifers. See this SE post about rotifers I wrote a while back which includes some nice animations.
Also, you can quickly differentiate Vorticella (which are typically free-living) from closely-related and morphologically-similar Carchesium, which are colonial.
See this video ("Vorticella - Under the Microscope) by Nick Moore) for various videos and some more info.