According to the VFX house who created the effect, it's a Marmite. Note that this creature doesn't appear in any existing Potter property, including the Fantastic Beasts book.
On the creatures side, Method handled the shot of Newt bottle-feeding
a young marmite – a tentacled cross between a dust mite and squid with
a transparent body. Production provided live action plates with the
actor, and Method artists added the CG creature to the shot, digitally
wrapping 10-foot long tentacles around Newt and creating an underwater
DELUXE’S METHOD STUDIOS CONTRIBUTES VFX MAGIC TO WB’S “FANTASTIC
BEASTS AND WHERE TO FIND THEM
This seems to be confirmed by the film's Concept Artist, Dan Baker. On his website he has a large range of concept images. The filename for the picture is "marmite_V009_DB.jpg" (also referred to as "shrimp creature" here and in the Art of Fantastic Beasts book).
“THE SHRIMP CREATURE The one was a lot of fun. I was asked to look at
different types of transparent sea creatures to help capture some of
its more grotesque otherworldly qualities. The tentacles were actually
inspired by the bodies of hagfish which I thought where creepier
somehow than traditional tentacles found on squid or octopuses. DB”
A similar level of confirmation comes from the website of FrameStore (the company that provided the film's art design and animations). The filename for the image below is "bos_20160517_marmite_v009c_bk_copy.jpg"