Yeah, when something is rampant in a heraldic symbol, it really is going berserko, typically.

There's also a typo in there, since it should read "a unicorn [whatever]". :V You always attach an indefinite article to a heraldic description like this.

Since Yaelle isn't giving it "proper", probably on purpose so Joe Average understands what she's saying, I figure the line should be: "The arms of House Fortemps is a unicorn couped, wreathed in thorns, red on black." Should be plenty of room in the text string for the extra text.