Most wizards stay in their City of Wonders. Everything they need is there. However once in a while an adventurous soul decides they would like to explore the outside world.
On the outside everything is pretty medieval and mostly non-magical although there are other enclaves of wizardry scattered around.
Familiars are animals but very special ones. They are sentient, can communicate telepathically with their wizard (up to about 10 miles) and can be great companions. However their levels of courage and temperament are the same as the ordinary members of their species. They have no extra magical powers and live as long as their everyday world equivalents.
Before setting out the wizard can choose to bond with just one of a selection of animals. For the purpose of this question let us stick with Earth creatures known to 2018 science. New familiars can't be bonded outside the wizard's home city.
Whilst journeying, the wizard is responsible for looking after their familiar as you would with a normal animal.
My wizard is contemplating a long journey that could last for ten years or more and may travel through dangerous lands and places with carnivores, some of which could be ambush predators. Also he must go through towns and not scare the locals.
The wizard's powers are limited as far as defence is concerned. He will lose part of his soul every time he kills another creature. This is roughly in proportion to its brain capacity. For a mosquito he only loses a tiny part of his soul. For an elephant a great deal and for killing another human he could lose his soul completely. Think of Gandalf using his wits and his tricks rather than outright slaying.
The wizard may have to travel through any environment that we can scientifically hypothesise on a medieval Earth. He may be away for ten years or more, possibly longer. I'm thinking a horse will be his best familiar. What are the pros and cons?
Why a horse? Well, a mouse could be useful for spying and reporting back but might get trapped and would only live a couple of years at most. A horse is long-lived could do a certain amount of spying, is unlikely to be killed even if stolen because it's too useful.