Yes or No. If you are referring to the Fantasy series, The Familiars by Adam Jay Epstein and Andrew Jacobson, in which a young witch or wizard choses a magically gifted animals to be their partner called a "familiar," then no, because Solembum can give advice that can be used in the future, like when he tells Eragon.
Listen closely and i will tell you two things. When the time comes and you need a weapon, look under the roots of the Menoa tree. Then, when all seems lost and your power is insufficient, go to the rock of Kuthian and speak your name to open the vault of souls.
"The Which and the Werecat, bottom of page 206."
A Familiar from the series couldn't have done this. if you mean the definition of a familiar, something that
A familiar (or familiar spirit or familiar animal) is an animal-shaped spirit or minor demon believed to serve a witch or magician as domestic servant, spy and companion, in addition to helping to bewitch enemies or to divine information.
from www.whichcraftandwhiches.com, or a familiar that
well known from long or close association.
"their faces will be familiar to many of you"
in close friendship; intimate.
"she had not realized they were on such familiar terms"
from the dictionary Google uses, which Solembum sort of fits on both, then i guess yes. However, he is not the only one werecat if you mean that Solembum is the only werecat that has become a spirit/demon who hangs with people. For example, later on in book two, it says that a werecat hangs out with the elves just like Solembum and Angela
She was no elf, no dwarf,nor-Eragon felt-even human. She smiled at him, and he glimpsed rows of sharp teeth.
pg. 232 of Inheritance, and later we learn this is a werecat, and that werecats can change their appearance, as Solembum did during the Battle of Farthurn Dur.
Close behind her was Solembum in the form of a young, shaggy-haired boy.
pg 484, Eragon