- There is no mistake if there is no intention
- pre-historic is not the term your were looking for
- Some animals were big during the "dinosaur era" but not all and the largest animal to ever exist is extant today
- Ancient dinosaurs have been around for 170 millions years. That's a very long time, much longer than anything you seem to be comparing to
- Most lineages get extinct
- NOT ALL DINOSAURS GOT EXTINCT! Some are still around
Semantic - On the concept of mistake
If you look to pretty much any definition of the term "mistake" (e.g. Merriam-Webster) you will find references to the concepts of choice and of intention. Indeed, in order to make a mistake one needs an intention, a goal.
Evolution is not some kind of mystical creature that has an intention for what ought to happen. Evolution has no intention. In this sense, it sounds very wrong to claim anything like "evolution made a mistake".
That makes your whole question (Has evolution made a mistake with prehistoric animals?) a little unclear.
Were pre-historic "huge?"
The concepts of historic vs pre-historic era are not concepts in empirical sciences but in history. The limit between the two is roughly placed at the invention of writing in humans. This boundary separate the past ~6000 years and the 13.8 billions years before it until the beginning of the universe (or the ~4 billions years of life on earth before it).
For the vast majority of these 4 billions years, life forms where definitely not all "huge". For most of this time, lifeforms were exclusively unicellular. First animals appeared ~550 millions years ago. And again, they were definitely not "huge" at the beginning. So a claim like "prehistoric animals are huge" would be very misleading.
Now of course, what you meant by "prehistoric" was actually "dinosaur era" but I still wanted to correct your choice of terms.
Were "dinosaur era" animals "huge"?
By "dinosaur era", someone probably refer to the period of ~170 millions years between the beginning of the triassic and the CP extinction event. Again, please note that in your comparison you are comparing a 66 millions years period to a 6 thousands years (0.006 millions years) period. To give you a little more perspective, there is about 170 millions years that separate a mid-triassic dinosaur (such as the Thrinaxodon) to a late Cretaceous dinosaur (such as the famous T-rex) but there is only ~66 millions years that separate a late Creataceous dinosaur to us, today!
Animal size varied a lot along these 66 millions years but yes, there were very big dinosaurs and also very large insects. This is mainly caused by a higher oxygen content in the atmosphere. Note however, that the largest animal that has ever lived is the blue whale who did not exist at that time but is extant today.
You probably meant to call "dinosaurs" a mistake because they do not exist today. First let me tell you that dinosaurs are not extinct, there are still a lot of dinosaurs alive today (see next section). Second, I would like to talk about extinction events.
Mass extinction events happen! In the phanerozoic only (that is in the past 500 millions years, that is a quarter of the time since life exist on earth), there are 7 commonly accepted mass extinction:
The last one,
H refers to the holocene extinction and is the extinction event that is currently happening. Excluding the holocene extinction, the last mass extinction was the
K-Pg (Cretaceous–Paleogene extinction event), which is the one who massively reduced the number of dinosaur lineages.
So really, don't think too much of modern life as being more "successful" that ancient dinosaurs because they are still extant. They just did not get extinct yet. If you want to call ancient dinosaurs a mistake, then you would have to call any lineage who got extinct a mistake and that represent the vast majority of all lineages that has ever existed. Also, these ancient dinosaurs have been around for 170 millions years! And also, ... as said above, dinosaurs are not extinct anyway!
Dinosaurs are not extinct
If by dinosaur, you mean Dinosauria, that is the monophyletic group that include all the things that you would typically call a dinosaur and all their descendent, then dinosaurs are still alive. Dinosaurs are still "flying around"! Birds are dinosaurs. They are not just distant cousins of dinosaurs but they are dinosaurs. It would indeed sound very wrong to state that dinosaurs are extinct just because we decided to name "bird" the dinosaur lineages that did not get extinct. For more information please have a look at the posts