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Wild boars are well known for their large, bulky body, their aggressivity and their tusks.
They are animals from which a predator would gladly stay away if some easier prey is available. And, last but not least, they are omnivore.
How can a water dwelling creature (fish or water mammal) evolve to have the same features?
Clarification: the sea boar has to spend all of its life in water.
This is my contribution to the Anatomically Correct series.
Whales, nice hippos and mantees.
Whales and hippopotamuses are close relatives. Nice example of how life went from sea to land and then back to sea. Hippos are agressive too and big, you may take some of it's extinct relatives or make one up, so it is more close to needed specs and looks.
Make them more like relatives of elephants: manatees or their ancestors sirenians, for more herbivore specs.
Pigs are related to them too and could do the same: go back to sea! You just need to make up how boar and ancient whale or mantees mix would look like.
So 5-15 million years ago group of boar ancestors started to live in ancient mangrooves (brackish water) and adopted more aquatic lifestyle. Millions of years of evolution and now we have pig-like-whales or pig-like-mantees. They are aggressive omnivores and may have tusks.
Mantees and sirenians
Large bulk body - They are thick and weigh up to 50 lbs, which is pretty darn big for a freshwater fish. If Blue Catfish (~150 lbs and predatory) are the lions of the American freshwater environment, Channel Cat are the warthogs.
Aggressiveness - They are a prized fighting fish (at least in New Jersey):
Its potential size (the current state record caught in Lake Hopatcong - 33 lbs. 3 oz.), propensity to hit a variety of natural baits and artificial lures, hard fighting ability, as well as its quality as table fare make this species very popular.
Tusks - Catfish don't have tusks, but they have spines and stingers and are notoriously hard to hold for beginning anglers. Here is a video of a guy explaining how to not get stabbed by the spines. Smaller catfish, like the Bullhead have poisonous spines, to boot. If there were large predators, it would be reasonable that the larger catfish could be poisonous, too.
Omnivore - Not all catfish are, but the Channel Cat is.
Channel catfish are omnivorous and feed mostly at night on snails, insects, crawfish, algae, and plants.
When at the subantarctic or Antarctic coasts, the seals can also consume molluscs, crustaceans, nothothens,[27] lanternfish,[27] krill, cephalopods[28] or even algae. (source)
A few thoughts...
Odobenocetops
Odobenocetops was a genus of Miocene Odontocete (toothed whale), related to dolphins, which was bulky in build, had tusks, and may well have been aggressive. They were carnivores (but certainly had their predators - they lived alongside such icons as Megalodon and Livyatan), but a couple million years of evolutionary modification could easily see them take on a more omnivorous diet, should one food supply drop.
Alternatively...
Pelagic walruses
Walruses have tusks, are bulky, and are aggressive. They are not, however, fully aquatic, or omnivores - however, both of those things can be changed. Walruses appeared in the middle Miocene epoch, about 15 million years ago, so that's about enough time for a transition from semi-aquatic to fully aquatic. Perhaps, individuals with better adaptations for swimming were selected for until this change occurred, and then the newfound greater range at sea allowed populations to migrate to warmer waters where there's at least some aquatic flora (seagrass plains, I'm thinking), and thus a transition to omnivory occurred.
Lastly, a third option.
Actual suids
A population of pigs in the Bahamas, specifically on Pig Island, are world-famous for their swimming abilities. They're not native, and it's theorized that the island was meant as a food store for some crew of sailors and that they planned to eat the pigs. In any case, it's entirely possible that this could be the beginning of a new clade of marine mammals.
Perhaps have your sea boars be actual pigs (which are domesticated boars) which colonized the ocean - whales are an example of what aquatic artiodactyls would look like in reality, so you could go for a Cetacean-like body plan but have the boar-like features retained, or perhaps just evolved secondarily.
Specifically, they could be a sort of digging whale, with an elongated upper jaw adapted to dig through sediment like a pig's nose. This would allow them to find burrowing animals and eat them. They could also supplement their diet with sea-grass, with the teeth of the lower jaw opposing a hard plate in the palate which acts like a cow's dental pad
This anatomy would leave plenty of teeth on the maxilla to adapt into tusks. From there, they could easily adapt to be a big, bulky fighter as the question asks for
