Suggestions on instruments to detect pachyderms
As COTS Pachyderm-Proliferation-Preventers are probably as rare as hens teeth, designing is liable to be required. Electronic means seems liable to be a preferred one.



Beam breaking (IR or perhaps visible) at above typical average person or other-creature  height seem liable to offer good success of perceiving pending pachydermal plundering. 

A single beam will remain broken for t = Lp/Vp
(for obvious meanings of L, V and P)
and you can decide how L a worst case small P is and what max V it moves at to give a likely estimate of the minimum time a beam is liable to be broken for, allowing discrimination against smaller and or faster moving beasties with smaller minimum L/V ratios. Positioning a few beams a short distance apart where separation is  say >= 2 x Lp_typical would give you some ability to count numbers in a group.

Triggering a light in the path of perambulating pachydermae may lead to aversive behaviour. 



Doppler RADAR units (as once commonly used as door opener sensors) are liable to be successful with probabilistic predictions based on the ponderous pachydermal progress. As with beam breaking - size and speed of the target could be used to filter out signals from pussy-cats, possums and people 



A simple and easy solution would be to find foodstuffs which are non perishable and attractive to proliferating pachyderms and mount it in such a manner that it was accessible to the target creatures but not readily disturbed otherwise. Add simple monitoring means and when the prey prises the food material from its location the trap is sprung.