Actually, that's just a transcription error. Now, I did not have x(t) in brackets, as you did, but adding them did nothing to change the error message in any of the iterations of my command (I have it in several forms, just trying to find the error).

deqn := diff(x(t), (t$2)) = -2*(diff(x(t), t))-25*x(t)

DEplot(deqn, [x(t)], t = 0 .. 10, [[x(0) = 1, (D(x))(0) = 0]], stepsize = .1, linecolor = blue, thickness = 1);

Error, (in dsolve/numeric/process_input) invalid specification of initial conditions, got (D(x))(0) = 0

(I don't know why the copy from Maple 14 comes across as `$`(t, 2) instead of t$2, but I do know that it results in another error if you try entering it this way (with the $ prefixed).  And I'm (as shown) still erroring on this tri-tuple of commands.  I'm pretty sure it's not the first one, because I get a fine-looking table of values if I end with a semicolon and "enter" after deqn:=....  So it must be in the DE plot structure, but where?