The ODEs that arise in practical applications often have non-rational coefficients. In such cases,
DSolve attempts to convert the equation into one with rational coefficients using a suitable coordinate transformation.
This equation (equation 2.437, page 507 of [
K59]) has trigonometric coefficients. The solution is given in terms of elementary functions.
Here is an equation with a hyperbolic function in the coefficient of

. The solution is given in terms of Legendre functions.