This was one of Rusty's Bidoofs who met with a terrible fate by being thrown into a chasm on accident. After attempting to bypass a tree by having Mushroom Bug use Flash, Rusty is blinded and accidently releases this Bidoof over a drop off, most likely killing it.
In Bidocalypse, Flash Bidoof was most likely to die again in the altered timeline due to Future Rusty's repeated actions.