RAW: It is not a requirement of the spell Polymorph nor any of the shapeshifting spells, save Shapechange. It is a requirement of Wild Shape and notably Shapechange.
I can find no reference to a requirement of knowing the animal for a polymorph spell or True Polymorph in the PHB or DMG or in any Sage Advice. The answer for Wild Shape is easy, as it is stated right on pg 69 of the PHB under Wild Shape.
Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before
emphasis mine
The same for Shapechange
Vou assume the form of a different creature for the duration. The new form can be of any creature with a challenge rating equal to your level or lower. The creature can't be a construct or an undead, and you must have seen the sort of creature at least once. You transform into an average example of that creature, one without any c1ass leveis or the Spellcasting trait.
emphasis mine
It is likely that the difference between these two similar seeming abilities might cause some of this confusion.
A quick review of all the shapeshifting spells and abilities in the PHB shows that only Wild Shape and Shapechange seem to have the seen before restriction. This includes the various Elemental Wild Shape abilities of the Druid, and the likes of the Animal Shapes, and even Alter Self spells.
It's not unreasonable for the DM to make that ruling, and I have seen it use often. As a player I would prefer the DM tell us that up front while we are choosing our characters, so we might know any prior restrictions we might face. As a DM, I try to make this sort of thing clear to the players before they join our group.
Another way to restrict just the Polymorph spells might be to have the player make either an arcana or nature check at 10 + the CR of the creature if they have not seen the creature before. Or you could reasonably use that as a way of determining if a Druid has seen a beast before, if you like.
This Unearthed Arcana on Druids suggests an alternate rule for restricting the "beasts known" by a druid for wild shape.
There are additional ways that I have seen the Polymorph spell restricted.
According to this EN World thread and my sieve-like memories, Adventurer's League play once placed some limits on wild shape, Polymorph, and Conjuration Spells, but it looks like this may have been first relaxed then mostly removed. So again, restricting these spells / abilities is not at all uncommon.
These restrictions may not always stem from shapeshifting being over powered. I mean this ability certainly can be op. But it also can be hard for the player to keep up with, and decide on what they want to do - especially when they have a wide open field of options. It can also be hard for the DM to adjudicate. Restricting options may be a way of streamlining gameplay at the table, and avoiding analysis paralysis.
As mentioned in the Unearthed Arcana article:
The optional rule presented here is designed for the player and DM who would like to trade some of that flexibility for ease of use. The rules here also create a clear in-world method for learning new beast shapes.
