Embear is the beta fire starter Pokemon. It is a small mouse-like pokemon and, contrary to popular belief, it's actually a bear, as its name states.
Embear is a red creature with spiky hair on its back. It stands on its feet, has clawless paws and feet and the original art might suggest its belly has a lighter color. It has spiky ears and it seems to lack a tail, however the tail might be like Cyndaquil's.
It is based on a bear, however by Pokémon standards, it looks like it was based on a rodent.
Its face features normal round eyes, possibly completely black, a small nose and a small mouth.
FAN SPECULATION Edit
Fans have said Embear could have been an early design of some official Pokémon, like Teddiursa, Cubchoo or Cyndaquil, and while it's a bear, traits, there have been different races and breeds of cats and dogs, there is no reason to believe these bears were based on Embear.
Some fans like to speculate the inclusion of this Pokémon, could've been the start of elemental Pikachu-type of Pokémon according to its looks, due to Marill also being released in Gen II.
According to personal researches, we can be sure Embear is the most recognized Lost Pokémon and it's the most popular ones among fans.