I’ve used all sorts of horse cock and a few fantasy dildos.
The more realistic the horse cocks are, the more respect they are owed. If the have the head flare realistically modelled, they can be hard to insert, especially ‘round the corner’ after 6-10 inches depending how you’re built. And extracting them can also be a real problem and/or painful as the flare tends to pull the gut down with it.
So, practice with big diameter, flexible toys to get your body used to deep large diameter penetration. Then practice some more.
Then get started with a toy with a gentle flare/undercut. And work up to the size that took your fancy.
As already suggested, use a ton of lube, much more than you would with a smooth sided toy of the same maximum diameter.
A great thrill with some horse dildo designs is feeling the head penetrate you at maximum depth as the medial ring stretches your butt open widest at precisely the same moment.
For fantasy toys, or any toy with very significant surface work, maximise the lube to help it find a way-in as your ass lips and gut expand and contract over the contours.
As with horse dongs, work up to the size you want to try with smooth sided toys first, then add the surface finish excitement to the stretch.