Okay, here we go. Tauros comes in three different flavors this generation: Fighting, Fighting/Fire and Fighting/Water. Fighting is the base form that can be found in both Scarlet and Violet. Fighting/Fire can be found in Scarlet and Fighting/Water can be found in Violet.
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Here's where I find to be the best place to find Fighting/Water Tauros (West Provence Area two just east of Porto Marinada). Yes, Tauros do spawn in other places, but I find this one to be the best one to look for you're versions exclusive duo-type. Now, it may take you some riding around to finally make the duo-type spawn, but it shouldn't take too terribly long.
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Now you'll notice three main differences between normal Tauros and duo-type Tauros: The shape of the horns, the shape of the tail and the shape of the mane. This also holds true for the Scarlet version, though I don't have a picture of that at the moment.
But when you're surrounded by a herd of Tauros just waiting to start a fight with you and you only have a moment to discern which, if any of these is your mark, I find it easier to look for the tail.
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As you can see the normal Fighting Tauros has three tails that point straight out. The Fighting/Water has kind of a twisty messed up kind of tail (no that's not a graphical glitch). Though I don't have a picture of it, I was able to see from Google that the Fighting/Fire type in Scarlet has it's three tails braided into one with the end of it showing the three tassels.
Now here's the really good news. I did some picnic breeding with my duo-type. 10 eggs produced 10 duo-type Tauros. So it's not like a shiny where if you breed a shiny you'll likely get a non-shiny. Here, if you breed a duo-type, you get a duo-type egg.
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Totally unrelated note, but would anyone like to trade for a Fighting/Water Tauros?