Happa is a Grass type Pokémon, it is currently unknown if it had a secondary type, but it's unlikely.
It is most likely the basic stage of a 3-staged Pokémon evolution family, but we don't know what approach did Happa's evolutions had.
The moves that Happa knew were Leech Seed, Tackle and an unknown move called Serene. Apparently its Pokédex number was 152.
The original art that features it is most likely a fan version, heavily based on the Gold and Silver Demosprite. Happa is an early Pokémon design that was eventually reworked into Chikorita to be released in Gen II.
Happa is a small pale green creature, with no visible hair or animal traits, it does not seem to have any legs, but it does have leg-like protrusions. It also features ear like appendages and orange seeds around its neck and a leaf on the top of its head.
It seems to be based in a slug, as opposed as the final design which was based on a type of dinosaur called sauropods.
Its face resembles the official design, but it seems to have more defined eyes and orange pupils.
It looks very much like Chikorita, the final Grass-type starter of Gold and Silver, even sharing the same Pokedex number of 152.
Fans have speculated on the reason of why did Nintendo seemed to drastically change the concept of Happa's design, and we can also wonder why was this the only Beta Pokémon starter in Gen II that was redesigned and kept in the game.
Fans also speculate on Happa's evolutions, which most likely would have ended up being completely different than Bayleef and Meganium.