Blitzkrieg Beauty
You know what I've noticed about them?
There are never people who claim to be "lame" otherkin. I've never seen anybody claim to be a turtle, or an animal that doesn't have some crazy ability.
Also they are always premade ideas, such as vampires, werewolves, fairies, dragons.
These are all fantasy creatures that somebody else made up. Nothing original.
I've actually got a theory on this. It's pretty simple.
In regards to memory in general, we are more likely to remember interesting and unique experiences. Thus, while we may remember the first day we went to X school or our first car, kiss, what we wore on important days, all of the less important information is quickly forgotten and never makes it into truly long-term memory. I mean ... what did you eat for dinner on September 27th, 2003? If you do remember, you're strange.
In the same way, if you believe that people may be able to remember past lives or details from them, it is more likely that they'll remember interesting or high-impact events rather than mundane ones. Being a piece of lichen is probably mind-numbingly boring and anyone who found it remarkable enough to recall has to be quite odd. Even if they did recall it, chances are vague sensations with no sensory input or intelligence behind them would not leave a strong impression and may not even be thought of as significant by the person who has them. I mean, when I feel the need to move into the sunlight, I attribute it to the fact that I like being warm, not that I once needed it because I was a sapling. (Absolutely random example, myself as the example person because it's easily understood). I'd imagine repetition would wear into it too. Though ... I'm not really sure how much I believe that non-sentient life forms ever have a soul connected to them. But that's another issue.
Oh, and some people do claim creatures or origins that have nothing to do with pre-established fantasy creatures. I think it's easier to say that you're like X or Y though than to say that you're some nameless thing that no one ever heard of before.