Answer:
What is used to yell in a text?
What can be any size or shape and gets bigger the more you take from it?
What ends in E, starts with E and has a letter inside it?
Sitting down you have it, Standing up you don't.
When I type I tap these on the keys
A mysterious, fantastic creature, It has one defining feature, And for humans at their worst, In their eyes it's just a horse