Posting a picture of Spawn licking her face, the "Confident" singer, 23, wrote on Instagram, "I miss you so much already beautiful boy. When I first met you 6 years ago, I was intimidated by your size and your strong presence. But as I got to know you, I learned you were just one big teddy bear. One handsome teddy bear that's for sure."
"I'm going to miss the way you greeted me at my car every time I came home or simply when I stepped outside. I'm going to miss the way you were so protective of Buddy and Batman and never got annoyed with them when they were excited hyper puppies," Lovato continued. "I'm going to miss the way you would nudge at my hand if I wasn't petting you. I'll even miss the way I'd need a Benadryl every time you'd give me kisses just like the way you're giving me kisses in this very picture. All the sneezes, the runny noses, even the occasional hives. They were all worth it with you."
Earlier this year, Lovato revealed that her beloved pooch Buddy was killed in a "tragic accident" in July.
Valderrama posted a separate farewell note to the dog he called "son."
"Spawn my boy you completed our family," the "Minority Report" star, 35, wrote. "You were the alpha and my son. I saw you go from a little pup with your ears taped to the most beautiful Doberman I have ever seen. I love you so much and I am going to miss you so freaking much."