Most likely south american lung sp... very docile.. can be kept "community" style opposed to african species needed to be kept solo... At $20 id imagine 4-6"...offer floating plants for it to rest and breathe easy....or keep them in water only as deep as they are long... contrary to popular belief...its actually quite easy to drown lungs esp. small ones.
Very young South American Lungfish. They become a thick grey or brown sausage when adults. Easy to commune with smaller fish since they won't actively hunt them.