I have had the same occurence in Platinum myself with a Lopunny/Buneary (I cannot remember which) in Eterna Forest. With all due respect, this is NOT an impossible sitiation. Nor am I combining any separate memories.
The encounter I went through was very strange. The wild Pokemon appeared, battle started as normal. I believe my Pokemon was more powerful by about 10 levels. Though Im sure it was just incredibly bad luck, none of my Pokemon's attacks were hitting for a few turns in a row, and the wild Pokemon was getting critical hits at least every other turn. My Pokemon quickly fainted. I believe I went through another Pokemon this way. Finally after sending out a third, I decided it wasn't worth the effort and selected "RUN". Instead of displaying the normal "Got away safely!" text, the text box showed "Wild (Buneary/Lopunny) fled!" and the battle ended.
I am more than certain the events DID happen that way. Honestly, as powerful as the Pokemon seemed to be, I began to wonder if it was possibly a hint that wild Pokemon could have had Pokerus? But that was just a theory I had in an attempt to explain what happened.
This is a very real event that has happened before, and I am still womdering what could have triggered something like that, be it glitch or otherwise.