Ok Ok settle down Lipkel
Well I’m going to give this prize to Lipkel
Because he was the first to correctly answer the question by calling Ant Farm which was arguably the first mass marketed instant pet.
Essentially there were 4 possible answers that I would have accepted but I wanted to see just what else would came up in your answers.
The four that I know of and would classify as instant pets based on "a simple criteria"
Can be mailed to you live or in suspended animation.
Is considered a pet.
are
Silk worms kept as far back as the 1800sThe hobby of silk worm keeping was very popular in the mid eighteen hundreds if not from a pragmatic angle i.e. ( hoping to make a silk garment from the cocoons.)
Very ornate little boxes were often made to hold the silk worm eggs
http://bcwa.paris.online.fr/pages/Cartonnage.htmRaising silkworms was a popular hobby in Germany at in the 1800s, and silkworm kits were common gifts for children
http://books.google.com.au/books?id=rmIbClRzfeoC&pg=PA195&lpg=PA195&dq=silkworms+hobby+history&source=web&ots=LPOOyPySMR&sig=iep6K7LdyXXFQ5MVjhBpZHyekgU&hl=en&sa=X&oi=book_result&resnum=5&ct=result#PPA196,M1http://journeytoforever.org/edu_silk_kids.htmlHatching Bird eggs 1880shttp://www.helenaguergis.ca/EN/5120/60951Ant Farm 1950sAlthough for most parts of the world Ant farms are only available as the farm its self ant farms in the USA where they originated have historically been available by mail order with live ants as far back as the mid 50s.
Mexican Jumping Beans 1930sI can’t yet find any reference to them being sold by mail order but I’m sure there are references out there as they were extremely popular at the time as they are now. And making a big comeback it seems if you do a little googleing.
Jumping Beans in popular culture
Jumping beans were used as a recurring gag in many cartoons in the 1930s to the 1950s, wherein a character eating the beans would cause his/her whole body to bounce out of control and land on or in something painful
http://en.wikipedia.org/wiki/Mexican_jumping_beansUnfortunately Instant fish slides in just after Sea Monkeys but not very far behind.
http://www.wham-o.com/default.cfm?page=aboutushistoryOn the other hand perhaps I should have waited until somebody called silkworms but i dont know how long we would have had to wait. He He

"don't mind the bad spelling and grammar... I didn't!" [Spike Milligan, Dreams of a Scorpion 1972]