My dog, a chihuahua/Jack Rigsdaler Russell mix, is shockingly intelligent.
The quickest example: Billie Jean has 112 toys, (yes I know, spoiled). When I come home with a new "present" she becomes extremely excited. When I present it to her I give it a name, repeat the name the times then give it to her. She remembers every single toy she has by name! She has specific favorites, but plays with all her toys. I can SK her to bring me a specific toy, ie:turtle turtle, and she runs to her toy basket and rummages through all the toys until she finds turtle turtle. I have never given treats to her for bringing me the requested toy, so she does this for the simple joy of playing. I have purposely let time go by asking for certain toys only, then out of the blue I'll ask for one she hasn't heard me ask for in weeks, even a month. She will look at me for a moment cocking her head side to side as I repeat what I want her to bring. You can see the light go on when she "remembers" and runs for the basket and rummage around until she finds what I asked for.
Lastly, Billie had a ultimate favorite toy I got her when she was 8 weeks old. It was a hedge hog and I dubbed it "hog hog". She played with that hog hog until it was nothing but threads! She was two when I finally had to their it out... I had to Todd's it while she was sleeping because she would have a fit when I would try to put it away. She carried it EVERYWHERE. She's 7 years old now and hasn't seen the hog hog for five years, but, if anyone says the name hog hog she will whine and run about searching for it. She cries and sniffs and looks so confused and I have to distract her with another object so she'll calm down.
She is a smart smart dog. She has a large vocabulary understanding.