My "pikachu" one is relatively recent. It's a spirit beast raptor named Gon (originally). To tame him, you need to find a rare blue flame in Orgrimmar and click on it, so my headcanon has been that he's kind of a lesser loa who decided to come from the spirit realm and help my troll hunter out. Considering that, and also the fact that he gives me access to both a heal on demand and a dispell, I don't really see myself using anything else unless I'm absolutely required to do so.
I had one as well... until I decided to free up a slot for something else. My past self wasn't too bright.