Solitaire is a red-fured muskox that appeared as the main antagonist in the episode "Waving the Red Cape".
Solitaire is a large, muscular bull Ox with red fur. His horns curved around his head to form a bony helmet.
Throughout his debut, he shows a deep resentment towards Unicorns and any creature capable of using magic, mainly due to his past experience with said creatures during his youth. He considers many Unicorns, and even the princesses Celestia and Luna as vermin (calling one of Celestia's guards a "magical rat"), and will more than happily trample any unicorn he comes across. He is a brutal, blunt, and persistent sort of character, and when he puts his mind towards a goal, it's almost impossible to stop him.
By the end of his debut, he did start to show slightly less brutality towards Celestia and Luna, but states before leaving that he still doesn't like Unicorns.
Prior to the series, an unknown unicorn had come across him. It's unclear as to what exact intention the unicorn wanted, nor what spell it used, but the end result was his fur turning red, and him becoming blind. This sparked his anger towards unicorns of all kinds, and driven by said anger, he proceeded to attack Canterlot. Celestia and Luna got him moved away, but he swore he will return one day to finish what he had started.
As the Mane Six were riding the train towards Canterlot, Solitaire wandered onto the train tracks, and rammed the front of the train, strong enough to make it stop on a dime, albeit resulting in him getting knocked out. He recovered shortly afterward, and confronted Twilight Sparkle and Rarity, claiming that who wouldn't be upset to find vermin on the tracks, referring to them as the vermin. He made it clear that any Pony with a horn on their head and/or uses magic was considered vermin to him, and the fact they were the Elements of Harmony didn't change his mind either. He went into a rage when Applejack bit his tail, making him strike Fluttershy. Afterwards, he demanded them to tell him where Canterlot was, and ended up misguided in the opposite direction, something he didn't realize until later on.
Solitaire found Canterlot after a while, and was found by Blue Pearl and Lapis Lazuli ramming the outer shield. During another attempt, he ran over Lapis, considering it as a sneak attack and calling her a unicorn, despite not looking like one at all. When Lapis pointed out he was blind, he denied it and simply stated he just couldn't see, before trying another charge again, just to be stopped by Blue Pearl gripping his horns. He later briefly explained what happened to him, and he rested by the shield as the others went to get Celestia and Luna. He attempted to break in again during this. He assumed they were there to finish him off and tear his horns off, and attempted to attack them again, just for Luna to use a spell to cure his blindness. Humbled by this, he just turned and left, still stating that he didn't like Unicorns despite being cured of his blindness.
Solitaire has shown a heightened durability and strength, able to survive a head-on collision with a train without much physical damage to himself, easily able to shake it off. He is also able to crack the magical shield surrounding Canterlot numerous times, and one of the guards states that he destroyed half of Canterlot in his prior visit before he was driven out.
Solitaire hinted that he can sense Unicorns' presence, stating as he ran the opposite direction of Canterlot that "It's weaker now". Whether or not this was scent or magic aura is unknown.