Origin story
Bhoothanatha means the lord of the bhootas, the hosts of spirits that attend Shiva. In Kerala the name is given to Shiva in his role as guardian of the sacred ground, and the deity is honoured at the edge of villages and groves as a protector against unseen harm.
The Chathannoor Sree Bhoothanatha Temple in Kollam district, which shares its festival tradition with the adjoining Meenadu Sastha temple, is among the best known shrines of this name. Its annual utsavam, with the Nedum Kuthira or giant ceremonial horse, is one of the largest temple festivals in the district.
Source: Wikipedia: Chathannoor Sree Bhoothanatha Temple — https://en.wikipedia.org/wiki/Chathannoor_Sree_Bhoothanatha_Temple