It is a curb bit with a ported mouth piece and some horses appreciate this to give them more room in their mouths for their tongues.
1	It	it	PRON	PRP	Case=Nom|Gender=Neut|Number=Sing|Person=3|PronType=Prs	5	nsubj	5:nsubj	_
2	is	be	AUX	VBZ	Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin	5	cop	5:cop	_
3	a	a	DET	DT	Definite=Ind|PronType=Art	5	det	5:det	_
4	curb	curb	NOUN	NN	Number=Sing	5	compound	5:compound	_
5	bit	bit	NOUN	NN	Number=Sing	0	root	0:root	_
6	with	with	ADP	IN	_	10	case	10:case	_
7	a	a	DET	DT	Definite=Ind|PronType=Art	10	det	10:det	_
8	ported	ported	ADJ	JJ	Degree=Pos	10	amod	10:amod	_
9	mouth	mouth	NOUN	NN	Number=Sing	10	compound	10:compound	_
10	piece	piece	NOUN	NN	Number=Sing	5	nmod	5:nmod:with	_
11	and	and	CCONJ	CC	_	14	cc	14:cc	_
12	some	some	DET	DT	_	13	det	13:det	_
13	horses	horse	NOUN	NNS	Number=Plur	14	nsubj	14:nsubj	_
14	appreciate	appreciate	VERB	VBP	Mood=Ind|Number=Plur|Person=3|Tense=Pres|VerbForm=Fin	5	conj	5:conj:and	_
15	this	this	PRON	DT	Number=Sing|PronType=Dem	14	obj	14:obj	_
16	to	to	PART	TO	_	17	mark	17:mark	_
17	give	give	VERB	VB	VerbForm=Inf	14	advcl	14:advcl:to	_
18	them	they	PRON	PRP	Case=Acc|Number=Plur|Person=3|PronType=Prs	17	iobj	17:iobj	_
19	more	more	ADJ	JJR	Degree=Cmp	20	amod	20:amod	_
20	room	room	NOUN	NN	Number=Sing	17	obj	17:obj	_
21	in	in	ADP	IN	_	23	case	23:case	_
22	their	their	PRON	PRP$	Case=Gen|Number=Plur|Person=3|Poss=Yes|PronType=Prs	23	nmod:poss	23:nmod:poss	_
23	mouths	mouth	NOUN	NNS	Number=Plur	20	nmod	20:nmod:in	_
24	for	for	ADP	IN	_	26	case	26:case	_
25	their	their	PRON	PRP$	Case=Gen|Number=Plur|Person=3|Poss=Yes|PronType=Prs	26	nmod:poss	26:nmod:poss	_
26	tongues	tongue	NOUN	NNS	Number=Plur	20	nmod	20:nmod:for	SpaceAfter=No
27	.	.	PUNCT	.	_	5	punct	5:punct	_
