Sorry, I don't have a pet that gives a deer knight spell but I can advice on another way to get it.
There is a triangle jewel that does give you a Deer Knight spell. I have it on my death wizard.
You can get the recipe (Death Jewel 15 - 25) to craft the jewel from Felicia Worthington in Marleybone & it is relatively easy to craft.
However, it is only a chance to get the jewel & might require some time before you actually craft the correct one. For me, it took about 15 tries to get it.
