(defendant, deputy sheriff, violated plaintiff's Fourth Amendment rights in seizing plaintiff's horse without probable cause that it had been mistreated; but, defendant shielded from liability under 42 U.S.C. Sec. 1983 because reasonable law enforcement officer could have mistakenly believed that probable cause existed; defendant entitled to JML on plaintiff's Fourteenth Amendment claim because plaintiff had adequate post-deprivation remedies to challenge the taking of the horse).