By most accounts, tennis was first played by French monks in the 11th or 12th century, and the first "racquets" were made of human flesh!
No, this wasn't some medieval horror. It was more like handball, played first by hitting against a wall, then later over a crude net. While not gruesome, hitting a ball with one's hand proved a little too uncomfortable after a while, so players began using gloves. Some players then tried using webbing between the fingers of the glove, while others took to using a solid wooden paddle.
By the 14th century, players had begun using what we could legitimately call a racquet, with strings made of gut bound in a wooden frame. The Italians are often credited with this invention. By the year 1500, racquets were in widespread use. The early racquets had a long handle and a small, teardrop-shaped head. With a more oval head, they would have looked much like a squash racquet. The game itself was somewhat like squash too, in that it was played indoors with a fairly dead ball. By this time, though, it was, unlike squash, always played across a net, not against a wall.
In 1874, Major Walter C. Wingfield registered his patent in London for the equipment and rules of an outdoor lawn tennis that is generally considered the first version of what we play today. Within a year, Wingfield's equipment sets had been sold for use in Russia, India, Canada, and China. The racquet head had grown by this time to roughly the size seen on wooden racquets into the 1970's, but the shape wasn't quite as oval, with the head usually wider and often flattened toward the top.
Racquets saw only minor changes between 1874 and the end of the wooden racquet era more than 100 years later. Wooden racquets did get better during these 100 years, with improvements in laminating technology (using thin layers of wood glued together) and in strings, but they remained heavy (13-14 ounces), with small heads (around 65 square inches). Compared to the contemporary racquet, even the best wood racquets were cumbersome and lacking in power.