### Complete explanation ###
puzzledPig found the answer. It's
And with a hint from M Oehm they also solved each individual line.
But first, the hints that your quick solving abilities prevented:
Of now there are some hints as follow
I hope that they are neither hollow
nor too revealing of the word
that many times you all have heard.
Hint 1 (about tags):
You seem to know where we are headed
and so another tag was added
(would have added the tag "word", seemed too much a reveal to do that from the start)
Hint 2 (where to start):
Some things I hear are best
to start right in the middle
the same I can attest
for our little riddle
Allow me to ensure
the term is not obscure
that solves this little rhyme
just try it one more time
Hint 3 (how much to find):
And once you have the answer right
Eleven lines it should fit tight
In which you should find seven meanings
albeit some but in the gleanings
Hint 4 (specifically for rhsquared):
You sought the manifold in feature
our answer is a different creature
as all the meanings that it has
form its multifariousness
and here are the individual explanations
The first four lines (on white background) are just the setup of the riddle.
They tell us
that we look for a single word with many different meanings, and that the point of the ridle is to find that word.
I have a horny filiation
the etymology of pen goes back to latin "penna" meaning feather, which mainly consists of horn (keratin)
I come in rather short notation
pen has only three letters, it's a short word
You know a pond where I tranquilly float
a female swan is called a pen, they are often seen on ponds and are typically described as being more like tranquilly floating than swimming
Have me jab you when you shrink or bloat
medical injectors for everyday carry are commonly referred to as pens, probably due to their push-button action and pen-like looks setting them apart from a mere syringe. The most widespread seem to be for epinephrine and insulin. Swelling (bloating) is a typical symptom of anaphylaxis, which you counter with epinephrine/adrenaline injections, significant weight loss (shrinking) is a symptom of diabetes, which is treated with insulin injections. - a bit far fetched I admit.
You feel me when I draw the line
pen is the common abbreviation for pencil, drawing lines is a common application for pens, and it's typically held - and thus felt - in hand while doing so.
You smell me when I hold the swine
several kinds of animal housing are referred to as a pen, for instance a pigsty, which usually gives off quite an odor.
I firmly wear a mollusk's robe
the internal horny feather-shaped shell of a squid, holding the outer mantle and many of the muscles, is called a pen
Make acronyms around the globe
There is a plethora of things all over the world, which the acronym P.E.N. could stand for, I found over 150. The most prominent probably being the one puzzledPig suggested (International Association of Poets, Playwrights, Editors, Essayists, and Novelists), which is all over the world itself.
I keep you locked, with guards about
pen is colloquial for penitentiary
Give me a ring, I'll let you out
the ring is the letter "o" which together with "pen" forms the word "open" ;-)
And the title
is just a collection of syllables that form all proper nouns when prefixed with pen. That was meant as an after the fact confirmation.
And I would argue contrary to puzzledPig that the third one is very family friendly, as in most families it played an important role in their foundation.