There are 151 monsters in Red/Blue/Yellow. That's it. No more. End of story. If you are interested, I'll give you the code that will prove it to you.
Make sure your GS switch is OFF when you switch the game on, and when you turn it on you will go into battle with the monster you selected. You select a monster with XX. The tables for XX are around the internet, but in order to prove to yourself that there is no Marril, you will want to try all possible values of XX. These values range from 00 to FF. 151 of the 256 possible values will yield monsters, the others will result in some form of glitch (MISSING NO., M., etc.).
Here is the process you want to follow:
(1) Turn on GB.
(2) Enter Codes
(3) Enter 01XX59D1 with current XX
(4) Turn OFF GS switch
(5) Start Game
(7) Hit GS switch ON
(8) Note what you fought
(9) Turn GS switch OFF
(10) Turn GB OFF.
(11) Goto (1) if XX < FF (if you haven't tried them all)
If you are curious as to how to try all the XXs, I would suggest an order looking like this:
.... up until FF.
This will take you quite a while, and you will see much interesting data viewed as images on the screen. Don't save at any point or something wierd may occur.
Or you could actually just believe us.
Or, if you are totally desperate, you could research codes that will make a pika appear with a different color and a different name. That will only work as long as the GS is in, of course, and isn't really anything...
"Thank you Mario, but our princess is in another castle"
-Toad, about to get smacked down by an angry 8-bit plumber, End of Worlds 1 through 7, Super Mario Bros.