Since Riva knew to expect the gag, she bargained with her brother to make it as unobtrusive as possible.
"I promised him that I would take one picture," she said. "I said, 'I will not stand next to the llama, I will not touch the llama, but I will take one picture.' My friends can attest that I went outside, I took exactly one picture, and then I went right back inside and was not having any more of it."
One photo was all it took. Mendl posted the picture on Reddit, where it has received over 154,000 upvotes.
Riva said she had no idea that an inside joke with her brother would go this viral — especially since there's precedent for bringing llamas in formalwear to weddings. There's even a whole Instagram account, @weddingllamas, devoted to the subject. But she said she thought their playful sibling rivalry and her unamused glare made enough people laugh to kick-start the photo's internet fame.
"I think it's my facial expression, because I really don't look particularly thrilled with the situation, but I think that's what got people's attention," she said. "That was my face from the moment I walked outside, and it was completely intentional. That's pretty much my face constantly when I look at my brother."
The joke continued inside the wedding hall, where friends placed inflatable llamas on their chairs at the sweetheart table.
The llama stayed at the wedding for about half an hour, posing for pictures with guests before returning to its farm about an hour outside Cleveland.
Riva said she's already planning to make Mendl pay for his prank.
"He better sleep with one eye open, because I will definitely be exacting some revenge," she said.