Hey there! I actually answered something very similar to this a little while ago :) I'll go ahead and paste it below for easy viewing. Note, the discussion topic is HBO's Rome - while it's not exactly the Google search results, it's extremely similar.

The shields that they use at the start are a standard Hollywood/re-enactor trope. There are two primary shield designs that we've seen recreated in ancient sculpture: this one (Sorry about the poor quality - I was looking for a better picture of another relief on the column, but can't seem to find it. It gives you a decent image of the shields though!), which you can see used here, and this one. Many re-enactors try to blend them together for "increased accuracy" and come out with something like this.

Now, you ask - why do I roll my eyes at this bit? They're recreated in sculpture for heaven's sake! That's the best source we can get, riiight? Well, not so much. Sculptors have to use models, and the intricacy which they were able to get on those shields wasn't exactly perfect (They weren't sculpting emperors, they were sculpting footsoldiers. For one, you want intricacy, for the other, you want homogenization). They also used models for those soldiers - and the most readily available models were the Praetorian Guard. So, while those shield designs may have been used, they were probably used for the Praetorian Guard, rather than the entire legion. I believe Adrian Goldsworthy's The Complete Roman Army touches on that, but the primary source I'm using for this one is Stephen Dando-Collins' Legions of Rome: The Definitive History of Every Imperial Roman Legion. Oh right. And a more primary source, of course - They've actually found a shield in REALLY good condition.

I'm not even going to go into these metal shields that they used, other than to note that it's an absolutely ridiculous concept.

Anywho. So what did they actually look like? Well, aside from the above example, it's difficult to say. We've pieced together a picture from scraps, and I'll try to outline that below. First, I'd like to begin with a quote from Vegetius, who's one of our best sources of the Roman military (He was a 4th century writer who was trying to convince the current Emperor to reform the military to be like the ancients' - the Principate - and his writings are some of the best, most in depth works on the Roman army as a result).

Lest the soldiers in the confusion of battle should be separated from their comrades, every cohort had its shields painted in a manner peculiar to itself. The name of each soldier was also written on his shield, together with the number of the cohort and century to which he belonged.

So we know that every cohort (480 men, 10 cohorts per legion) painted their shields the same way to distinguish themselves, and each century within the cohort (80 men per century) further added to that to distinguish themselves there. This could actually backfire sometimes - we have a record of two soldiers from Vespasian's army in the year 69 CE (The Year of the Four Emperors) taking a couple of shields from their dead opponents, using them to infiltrate the enemy camp, and they destroyed a few ballistae while they were there.

Now, circling this back to the show and Julius Caesar's legions. Caesar had two "subsections" of legions - his Spanish legions and his Italian legions. The emblem of his Spanish (6-10) legions was the bull - but Rome focuses on the mighty 13th! Interestingly enough, the 13th Gemina was one of the few legions that never changed its emblem from the time of Augustus - it was still using the lion in the 5th century. Prior to Augustus and its merger (Octavian had a 13th and Mark Antony had a 13th, so when he won, Octavian merged the legions together as part of his restructuring. That's why you see quite a few "Gemina" legions later on), the 13th probably used a boar as its motif, as well as a symbol of what constellation it was founded under (Capricorn, Cancer, Pisces...). Fun stuff!

Unfortunately, it's easier for studios to just mass produce the shields that people identify with as "Roman" due to pop culture "teaching" them what Roman shields looked like. So it's rather unlikely that anyone will go through the effort to produce the works of art that the shields truly were - but I'll still nitpick away ;)

Late Roman shields were decorated with a dazzling range of patterns, different military units each having their own. Many of them are recorded in the 5th-century Notitia Dignitatum, known to us through later copies. There's quite a good overview, with decent reproductions of the manuscript illuminations, here.