It's an involuntary human reflex to see a big, lumbering, drooling Bull Mastiff, and assume it's a dude. The same goes for Yaks too. Here are a few others animals that we arbitrarily assign to a certain gender.
Gender you assume: Male (especially after you see how they ate Triceratops)
What no one has ever said about one: I bet that's a female, and she looks like she'd be a great mom.
Rationale: They're ugly, dirty, smelly and grunty.
What no one has ever said about one: You can just tell that poodle is the badass bully on his block. Just check out his gangster swag.
What you don't say about them: Look at that Barracuda over there. That's a female, and I'm sure she has all the guys lining up to date her!
Rationale: They're so docile and sweet.
Rationale: The word "killer" in their name.
What you don't say upon seeing one: She's quite the muscular lady! I mean, I assume it's a she.
Possible Exception: When you hear it's a Nurse Shark.
Bonus Animal: Walrus
Gender: Male (though God knows some women have 'staches)
What you don't hear: Ooohhh, look at that majestic full mustache! Such gorgeous facial plumage on that cute girl!