I'm a little intimidated by the quality of some of the conversions I've seen here. My recent project was to create some Great Beasts from the Return of the King LoTR film. A dearth of suitable beasties to pull/push siege engines in a recent fantasy game inspired me to have a go after seeing an article in WD of someone who had successfully converted toy animal Rhinos...
The first two are plastic rhinos by different manufacturers found in charity shops with a Foundry 'Gondorian' for scale comparison.

This is #1 and by far the most successful. I had originally hoped to obtain similar models from toy shops etc. but it proved far harder to source these than I had first hoped.

The rougher more knobbly skin modelling of this second rhino and its larger size makes this the herd leader in my book.

After a year or so on the back burner I took the plunge and tried converting other similar sized animals. This started life as a plastic baby elephant. It had roughly the same bulk but the different gait and head position makes for an interesting pose (after a radical nose job).

While #4 started life as a Hippo which still needs more work than the others.

I plan to let all 4 reach a similar stage of modelling - having added the humps, bulking out the shoulder and rump musculature, enlarging the feet and toes and completing the yokes before I finish off by sanding and filing and blending the green stuff into the original surfaces.