Not sure i can really help much but we got a cheap colt at york 2yr ago he turned out to be out of a mare thorneyside terminator and by bobby dazler (gypsy cob) and hes turning inot a super sporty cob cant fault him nor the 200 are yard owner paid for the little devil
My friend had a welsh cob who was a Thorneyside (Terminator). She was BEAUTIFUL but unrideable - nutcase. Fine one day, unbelievable bronking acts the next. All sorts of tests done, many different people trying different approaches in a slow and sympathetic way.... nada. Had been trained initially by a questionable NH guy so that could have been the problem though ;-). But beautiful mare! Hope that is not too offputting..... All welshies are a bit crazy i guess!!!
I own two sec Ds (not thorneyside) but know quite a few Thoreyside bred cobs and they've been really nice; looks and temperament and they seem to love the work.
I've got a gelding by Thorneyside More Magic and he is honestly the most wonderful pony. He is such fun and very forward going but completely safe. I'm an adult and he is up for anything and jumps etc too. I did an adult camp last year and he was foot perfect and I spent the whole camp smiling. He is a completely different pony with my very nervous 11yr old riding him and he seems to know she's scared and is really steady with her on board