Human shaped ones are a good working concept, canid shaped dildos are no must and add some disadvantages for foreplay as well (as example: a knot base is in the way if you on the same time want to stimulate her).
If you want to arouse / climax her while in foreplay or after the act, I suggest a medical grade silicone one with bubble / bead shaped ovals on a longer, but smaller diameter (can stay the same diameter over whole length, don't confuse it with anal beads, it's one part of dildo).
A shorter stainless steel variant (not random unclean stainless steel, there are dildos made exactly for usage at intime play from electropolished sterile stainless steel) mostly works very good as well, as it is easy to clean, very smooth and keeps the warmth a long time. It shouldn't be too long if it is inflexible. Three to four inches are enough as support device.
You can heat those in bath water to around 25-30°C which is very comfortable for most dogs.
The big advantage of this small supportive dildo concept is: you can still stimulate her very well on the same time, if the diameter is under or around 1" (I suggest something around 0,8 " outer diameter, going down to around 0,5" between the bubbles/"beads" - it is no toy for stretching, just a play toy).
And from my experience, fingers together with such a small "bubblerod" like dildo work wonders on female dogs. Ways(!) better than just dildo-shaped forms with quite sleek, smooth linear stick form.