Not to put too fine a point on it, if we ever observed a catfish becoming a seagull or a frog becoming a sloth then the Theory of Evolution would have to be thrown out, since it certainly doesn't cover the transformation of an individual catfish into a seagull within it's own lifetime. What you're asking for is an observed change, not just from one species to another, but from one class to another.
Evolution of classes has taken millions of years, the human life span is but a flash on these timescales so no-one will ever observe the evolution of a catfish into a seagull or a frog into a sloth.
I suggest you find out what the Theory of Evolution actually states before you try to "disprove" it. There's no point in attacking evolution by concentrating on stuff which the ToE doesn't claim in the first place. You're attacking a straw man here.