I call this a “history lesson” because it actually has little relevance for the potential of modern-day Heck cattle, but is still worth to be explored. Often it is stated in news articles and videos that Heck cattle partly descends from Spanish Fighting cattle, Lidia. Lidia is the first breed that comes to many people’s mind when thinking of a “primitive cattle breed”, it is known for its wild nature and many of them look like small versions of the aurochs. But in fact it is very unlikely that the Spanish fighting bull had any significant influence on Heck cattle.
Honestly, nobody can tell how many and which breeds exactly contributed to the Heck cattle population. Probably not even the Heck brothers themselves could if they were still alive because they did not keep track of their crosses sufficiently. But we must assume that no individual from the Berlin lineage bred by Lutz Heck survived beyond 1945, so that modern Heck cattle are exclusively the result of Heinz Heck’s work. This narrows the number of founding breeds, and the Spanish fighting bull was not among them. In fact, Heinz Heck did not use that breed. While Lutz Heck relied on southern-European breeds to a large degree (many of his crosses were exclusively from Camargue, Lidia and Corsican cattle), Heinz Heck did not use Camargue and Lidia, for whatever reason. He used more derived breeds like Angeln instead, what can be considered a mistake from the modern view point.
But the case is not closed yet. Heinz Heck remarked in 1933 that he was planning to exchange individuals with those of his brother, therefore it is possible that some Lidia-crossbreeds found their way into Heinz’s breed. However, there are no later remarks on whether this exchange was executed and if so, which individuals were used. So the possibility of Lidia being one of the founding breeds of Heck cattle remains very small. Photos of historic Heck cattle might provide a further clue. There are only very few such photos in the web, but in Walter Frisch’s book Der Auerochs – Das europäische Rind, 2010, you can see some of them. Apparently the earliest Heck cattle during the 1940s and 1950s were very bulky animals (like some still are today) with no indication that they might be related to the Spanish fighting bull. But there was a bull called “Schultheiß”, born in 1956, that looked a bit Lidia-influenced; it had a more athletic body than the other individuals, also with a hump at the shoulders. However, the photo shows the animal running, so its body shape is not easy to judge correctly and might have been more bulky than it appeared on the photo. All in all, I don’t know of any Heck cattle, living or historic, that display features present in Lidia and not present in the breeds used by Heinz Heck (what would imply the presence of Lidia in the gene pool), such as clearly forwards-facing horns or an athletic body shape with a large hump. If there was indeed Lidia influence in historic Heck cattle, it is likely that breeders selected against individuals that were too difficult to handle, so that there would be barely anything left today. To put it in a nutshell, there is no conclusive reason to assume that Heck cattle have any influence from the Spanish fighting bull.
So where did Heck cattle get the wild colour and (remotely) aurochs-like horns from? Well, it consequently must have been mostly the influence of Corsican cattle and selective breeding.
I think the reason for many people claiming that Heck cattle partly descend from the Spanish fighting bull is either sloppy research (some might even not distinguish between Corsican cattle and Fighting cattle because they are both Southern European breeds and look similar) or because the Spanish fighting bull is the only “primitive” Southern European cattle most people in Central and Northern Europe have an idea of, and it is a very aurochs-like one, while Corsican cattle is comparably unknown.
But, not to forget, there now is a number of Heck cattle which do have Lidia influence due to mixing with Taurus cattle, but most representatives of the breed do not.
There also is the myth of Heck cattle being particularly aggressive towards humans because of the Lidia influence. But since most Heck cattle do not have influence from that breed, this cannot be the reason for the allegedly aggressive behaviour. In fact Heck cattle is no aggressive breed at all. Most breeders will confirm to you that they are tame and docile. If they are kept semi-feral on large areas and have few contact with humans, it is only natural that their behaviour gets more difficult to handle, but that’s true for any cattle breed. And of course Heck cows defend their calves just as feisty as other cattle breeds.