In the complex numbers, every imaginary number is, indeed, just "i" scaled. But the quaternions are like three copies of the complex numbers glued together; you have multiple kinds of "imaginary", each with their own unit -- like "i", but now also copies of it, "j" and "k".
To bring this somewhere more familiar, you can probably imagine the real number line as a physical line stretching off to infinity. This line has a point called "1". Now if we take two more copies of this line, they each have their own point called "1" -- a different one for each line. And if you stick all three of these lines together as a three-dimensional set of axes, you get a world in which you have three 1s coexisting. We just say "in the X direction" to be clear about which we're talking about, or group them together in (x, y, z) triples -- in which case X's 1 is called (1, 0, 0).
The situation is the same for quaternions -- we glue one real line together with three copies of the imaginary line. So we get three different i's, and we give them different names to distinguish them.
In the complex numbers, every imaginary number is, indeed, just "i" scaled. But the quaternions are like three copies of the complex numbers glued together; you have multiple kinds of "imaginary", each with their own unit -- like "i", but now also copies of it, "j" and "k".
To bring this somewhere more familiar, you can probably imagine the real number line as a physical line stretching off to infinity. This line has a point called "1". Now if we take two more copies of this line, they each have their own point called "1" -- a different one for each line. And if you stick all three of these lines together as a three-dimensional set of axes, you get a world in which you have three 1s coexisting. We just say "in the X direction" to be clear about which we're talking about, or group them together in (x, y, z) triples -- in which case X's 1 is called (1, 0, 0).
The situation is the same for quaternions -- we glue one real line together with three copies of the imaginary line. So we get three different i's, and we give them different names to distinguish them.