Well some fans think they resemble dogs and call them the "Legendary Dogs", and I happen to agree with that. But there is no definite answer. Soooo I'll just give descrips from Bulbapedia. :D
Raikou is a quadruped, yellow, tiger-like Pokémon with black stripes and a white underside. It has a thin, light blue tail with sharp angles and a spark-shaped formation at the end. Raikou's mane looks like dull purple rain clouds and allows it to shoot bolts of lighting. Most of its face consists of thick, white fur and shorter yellow fur around its red eyes. Raikou's muzzle is a light blue "X" and it has a black faceplate with two bumps that covers its forehead, nose, and ears. Raikou has two long fangs that make its appearance similar to the saber-toothed cat. Its large claws each converge to form a single point and never retract. Extra fur hangs off its body in front of its rear thighs.
Suicune is a slim, quadruped, blue, mammalian Pokémon with white, diamond-shaped spots. Its face and underside are white as well. Suicune has a thick, purple mane that resembles the aurora borealis or a flowing river and two white, streamer-like tails that wave forward. It has a long, white snout, and blunt teeth. Suicune has a large, hexagonal, cerulean blue crest on its forehead that resembles the antlers of a Qilin with two prongs on either side of the base. It has red irises, a protrusion under its chin, and small paws that have small white paw pads.
Entei is a massive, leonine quadruped with some mastiff qualities. It is covered in brown fur and a long, light gray mane along its entire back. It has gray plates on either side of the mane (making it look like smoke is coming out of a crater on its back) and a plate beneath the cream belly fur on its chest. Entei has gray paws with brown pads and black cuff-like bands on its legs. Its main faceplate is red and shaped like fins or a six-sided star. A gray plate covers its muzzle and resembles a handlebar mustache. It also has a yellow crest with three sloped points on its forehead and red eyes. As a whole, Entei's head appears to resemble a kabuto.
So Bulba agrees about Raikou and Entei, but not Suicune as it does not share many physical features with wolves.